Adding an abstract dynsys class

[vkotaru]

Thank you @ChoiJangho for the repo. Hopefully, this will be the start of many collaborations to come.

As @ayush-agrawal mentioned shall we add? an abstract dynsys class (containing useful functions, for instance, all the ways to integrate the system 1. continuous integration, 2. discrete integration, 3. zero-order hold integration).

(Also, using this issue to get the lab started on using issues (hopeful other Github features)).

[ayush-agrawal]

Yeah, I can add in all of those functionalities. This can come from another repo that we can add as a submodule to this one.