Consider Including Sparta as a Submodule

[kathlenemagnus]

Currently Atlas supports 2 methods of including the Sparta lib. Option 1 is to use the Sparta library installed in the user's Conda env. Option 2 is to provide Atlas with the path to the Sparta repo and building it with Atlas.

Since the Sparta library itself doesn't take too long to build, we should consider just including Sparta as a submodule. Need to investigate whether there are any downsides to this approach.

[knute-mips]

😬

I'd discourage this for a few reasons:

1. Sparta shouldn't be changing that frequently. Not sure why this is necessary...
2. This could potentially cause divergence between a user's installed version of sparta, Atlas' version of sparta, and if folks get into the habit of "submoduling" sparta, there will be LOTS of versions floating around. As a sparta developer this terrifies me
3. This will increase build times for Atlas (albeit Sparta should be built once and with ccmake this should be painless). But still, that's a lot of disk space being used for common stuff

Instead of a submodule -- maybe an external project?