Move guassopt to optics/guassopt

Correct import in test_gaussopt

Correct imports in docstrings
Correct implicit import
Change location of docs
Remove merge conflicts
Add deprecation warning for sympy.physics.gaussopt
Change Google code to GitHub in SymPyDecprecationWarning docstring
Add issue number(7659) in the deprecation warning
Do not test duplicate gaussopt.py file
Skip physics/gaussopt.py for doctest
Docs of all system of ODEs implemented

This PR contain docs of all system of ODEs implemented till now.
Added a test case in @XFAIL

A test case is failing in `./bin/test` due to matching failure. The issue for this is here(https://github.com/sympy/sympy/issues/7723) and discussion for this is here(https://github.com/sympy/sympy/pull/7678/commits)
Corrected small things
Updated comment
Merge pull request #7678 from KDMOE/sysode_docs

Sysode docs
Merge pull request #7607 from debugger22/move-gaussopt

Move guassopt to optics/guassopt
Fix infinite recursion for NegativeInfinity when doing mod with a number
Merge pull request #7743 from debugger22/noo-max-recursion

Fix infinite recursion for NegativeInfinity when doing mod with a number
roots allows trig keyword: return cubic roots in terms of cos