Remove non-essential sim_manager features from halotools

[aphearin]

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.