Additions to groups.py: SL, PSL, random symmetric subsets, new Group constructor

[mittaltushant]

Added new module to generate linear groups. Abstract.py has a new method to create random symmetric subsets, and a Special Linear Group class.

[perlinm]

@mittaltushant I think I made all of the changes and additions that I had planned for this PR. Please review. If it looks good to you we can write unit tests. (Normally unit tests would be written before code review, but in this case I want to make sure we're happy with the general classes/functions/etc).

Notable changes/additions:

• Groups.from_generating_mats
• Groups.random_symmetric_subset
• SpecialLinearGroup class, with an added lift to d x d matrices
• ProjectiveSpecialLinearGroup class. The old construction has been removed because it was broken, but the new construction with Groups.from_generating_mats works.