Added system.py to physics/mechanics
Added system.py to physics/mechanics

Added a class called SymbolicSystem to physics/mechanics to act as a
container class for the equations of motion and other details related to
dynamic systems. I have created all of the class attributes and
docstrings.

postvakje/sympy,yashsharan/sympy,yashsharan/sympy,sampadsaha5/sympy,skidzo/sympy,kaushik94/sympy,aktech/sympy,souravsingh/sympy,madan96/sympy,jaimahajan1997/sympy,chaffra/sympy,postvakje/sympy,sampadsaha5/sympy,jaimahajan1997/sympy,aktech/sympy,madan96/sympy,jaimahajan1997/sympy,skidzo/sympy,chaffra/sympy,kaushik94/sympy,souravsingh/sympy,aktech/sympy,souravsingh/sympy,skidzo/sympy,yashsharan/sympy,chaffra/sympy,madan96/sympy,kaushik94/sympy,sampadsaha5/sympy,postvakje/sympy