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

Now SymbolicSystem can be called from sympy.physics.mechanics

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