Remove non-essential sim_manager features from halotools

Currently, there is a lot of functionality in `sim_manager` that is a pain to maintain. So in v0.7, some of this will be migrated to a separate "convenience package" that is not part of halotools.

The following list summarizes the features that I'm proposing _will remain_ in Halotools:

* **TabularAsciiReader**
* **UserSuppliedHaloCatalog**
* **UserSuppliedPtclCatalog**

The following list summarizes the features that I'm proposing _will be removed_ from Halotools:

* **CachedHaloCatalog** - This means that it will no longer be possible to use halotools to load a simulation from its nickname. Instead, Halotools will just implement a barebones read using h5py.
* **Automatic downloading** - The download scripts will be removed, so that users will need to use wget themselves. This will remove external library dependencies upon `bs4` and `requests`.
* **Automated bounds checking** - The code currently does a lot of automated checks to make sure the catalog has been properly formatted. All of this will now be punted to the user. This will make it possible to relax many current requirements, such as the current requirement that every column name begin with `halo_`.
* **RockstarHlistReader** - This is really just a wrapper around `TabularAsciiReader`, which will remain.

Most (though not necessarily all) of the removed features will be migrated to a separate location that, in practice, users will need to maintain themselves. The timescale for this proposed migration is ~December 2019, so anyone with particular needs for these features should weigh in soon.