Term.get_coefficients fails for jax implementation when built from source

Hi @dfm,

Today I've built celerite2 from source following the recommendations on the install docs. I'm trying to do something simple, like this
```python
from celerite2.jax import terms

sho = terms.SHOTerm(S0=1, w0=3000, Q=0.6)
sho.get_coefficients()
```
but I'm getting the following error
```
---------------------------------------------------------------------------
NotImplementedError Traceback (most recent call last)
Cell In [1], line 4
1 from celerite2.jax import terms
3 sho = terms.SHOTerm(S0=1, w0=3000, Q=0.6)
----> 4 sho.get_coefficients()

File ~/git/celerite2/python/celerite2/jax/terms.py:36, in Term.get_coefficients(self)
35 def get_coefficients(self):
---> 36 raise NotImplementedError("subclasses must implement this method")

NotImplementedError: subclasses must implement this method
```
At first I thought this could be an accident of the multiple `SHOTerm` implementations, for example, here

https://github.com/exoplanet-dev/celerite2/blob/e75dd45ca4f033b22d3ca19d0545a097c7441495/python/celerite2/jax/terms.py#L473-L478

and here

https://github.com/exoplanet-dev/celerite2/blob/e75dd45ca4f033b22d3ca19d0545a097c7441495/python/celerite2/jax/terms.py#L481

but commenting the first one out doesn't solve the problem.

Any ideas? Thanks!