Unable to access pymc3 submodule

Hi folks, I just discovered this updated version of celerite and am excited to add `celerite2`'s `PyMC3` GP into the [`Eureka!` ](https://github.com/kevin218/Eureka) package (as I'm finding more and more cases where using a GP would be beneficial). We already have the original `celerite` GP working (at least kinda) as well as the `george` GP for standard python minimizers and samplers (`emcee`, `scipy.optimize.minimize`, `dynesty`), but we also have a `PyMC3` version of our fitting code which allows one to use `starry` (ideal for eclipse mapping) and in general `PyMC3`'s NUTS sampler has allowed for much faster fits (for me at least). I know that `jax` and `PyMC4` offer advantages over the deprecated `PyMC3` and `theano` implementation, but over the past year I've already spent more than a fifty hours on getting `PyMC3` versions of all our astrophysical and systematic models implemented, I need to be able to use `starry` for the astrophysical model, and I'm looking for a fairly quick way to get GPs implemented. I was about to try `PyMC3`'s built-in GP, but I saw on the old `exoplanet` docs that `exoplanet` had a faster GP implementation for `PyMC3` sampling and then noticed that the code had been migrated here to `celerite2`.

However, by default I am unable to access the `pymc3` submodule of `celerite2` that is mentioned in the documentation. I've tried installing the version on main branch on GitHub (seeing that v0.2.1 doesn't have pymc3 code), but I get an error that `'celerite2' has no attribute 'pymc3'`:

```
> pip install celerite2[pymc3]@git+https://github.com/exoplanet-dev/celerite2
> ipython
Python 3.9.7 | packaged by conda-forge | (default, Sep 29 2021, 19:23:19)
Type 'copyright', 'credits' or 'license' for more information
IPython 8.14.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import celerite2
c
In [2]: celerite2.__version__
Out[2]: '0.3.0rc2.dev11+g74b0705'

In [3]: celerite2.pymc3
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[3], line 1
----> 1 celerite2.pymc3

AttributeError: module 'celerite2' has no attribute 'pymc3'
```

Looking at the package on GitHub I see the pymc3 folder under python/celerite2/pymc3, but it seems it isn't being imported in the `__init__.py` file. Just adding `from celerite2 import pymc3` to the `python/celerite2/__init__.py` file wasn't enough to solve the problem either, giving me the following error message when installing:

```
Building wheels for collected packages: celerite2
Building wheel for celerite2 (pyproject.toml) ... error
error: subprocess-exited-with-error

× Building wheel for celerite2 (pyproject.toml) did not run successfully.
│ exit code: 1
╰─> [27 lines of output]
running bdist_wheel
running build
running build_py
copying python/celerite2/__init__.py -> build/lib.macosx-10.9-x86_64-cpython-39/celerite2
copying python/celerite2/celerite2_version.py -> build/lib.macosx-10.9-x86_64-cpython-39/celerite2
running egg_info
writing python/celerite2.egg-info/PKG-INFO
writing dependency_links to python/celerite2.egg-info/dependency_links.txt
writing requirements to python/celerite2.egg-info/requires.txt
writing top-level names to python/celerite2.egg-info/top_level.txt
reading manifest template 'MANIFEST.in'
warning: no directories found matching 'c++/vendor/eigen/Eigen'
adding license file 'LICENSE'
writing manifest file 'python/celerite2.egg-info/SOURCES.txt'
running build_ext
clang -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -fwrapv -O2 -Wall -fPIC -O2 -isystem /Users/tjbell1/miniconda3/envs/eureka_starry/include -fPIC -O2 -isystem /Users/tjbell1/miniconda3/envs/eureka_starry/include -I/Users/tjbell1/miniconda3/envs/eureka_starry/include/python3.9 -c flagcheck.cpp -o flagcheck.o -std=c++17
building 'celerite2.driver' extension
clang -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -fwrapv -O2 -Wall -fPIC -O2 -isystem /Users/tjbell1/miniconda3/envs/eureka_starry/include -fPIC -O2 -isystem /Users/tjbell1/miniconda3/envs/eureka_starry/include -Ic++/include -Ic++/vendor/eigen -Ipython/celerite2 -I/private/var/folders/f3/_h5lxm511fv9sjb5d3xrj0jm0000gq/T/pip-build-env-u9cav0rq/overlay/lib/python3.9/site-packages/pybind11/include -I/Users/tjbell1/miniconda3/envs/eureka_starry/include/python3.9 -c python/celerite2/driver.cpp -o build/temp.macosx-10.9-x86_64-cpython-39/python/celerite2/driver.o -std=c++17 -mmacosx-version-min=10.14 -fvisibility=hidden -g0
In file included from python/celerite2/driver.cpp:6:
In file included from python/celerite2/driver.hpp:8:
In file included from c++/include/celerite2/celerite2.h:4:
In file included from c++/include/celerite2/core.hpp:4:
c++/include/celerite2/forward.hpp:4:10: fatal error: 'Eigen/Core' file not found
#include
^~~~~~~~~~~~
1 error generated.
error: command '/usr/bin/clang' failed with exit code 1
[end of output]

note: This error originates from a subprocess, and is likely not a problem with pip.
ERROR: Failed building wheel for celerite2
Failed to build celerite2
ERROR: Could not build wheels for celerite2, which is required to install pyproject.toml-based projects
```

I was able to find the referenced `Eigen` package at https://eigen.tuxfamily.org/, and I downloaded the latest version and put the entire contents of that package in `celerite2`'s (previously empty) `c++/vendor/eigen` folder. With that package downloaded and my edit to the `__init__.py` file, I'm now able to import and use `celerite2.pymc3`.

I thought I'd document my troubleshooting process here to help anyone else trying to do the same thing or to help the devs figure out what changes need to be made to the documentation and/or package.