Code cleanup, pep8 fixes, docstrings edits

- Code cleanup with flake8 for pep8 fixes in new code
- Edits to docstrings
- Fixed doc tests
Add feature to initialize medium with refractive index. [skip ci]
Import constants like permittivity of free space, speed of light in vacuum and magnetic permeability from sympy.physics.units rather than using Symbols for them.
Add tests for medium
Add args test for Medium
Added tests
Add note for the commented failing test
Add spatial component in representation
Add rewrite as exp function
Add more tests
Fix for python 2.6 - dict comprehension changed

Using a generator expression and the dict() constructor now.
Changed the __eq__ method and added the equal method
Rectified bugs, made improvements in __new__ code
to update
Merge branch 'master' of https://github.com/sympy/sympy into basicvector
Small change
Merge pull request #7546 from debugger22/wave-equation

Enhancing TWave
Code cleanup
Merge pull request #7331 from jksuom/floats

Floats - neither rational nor irrational
Added doctests
Merge branch 'master' of https://github.com/sympy/sympy into basicvector
Rectified doctests
Shift two issue numbers in strings like issue([0-9]+), see faedd55

I'm not sure about the #4021 (old issue number 922), maybe
it's #4087.  This issue number looks as a typo for me (998 instead
of 988 in the old bugtracker).  Test was added in the #4021 (commit
a945bc6), but it works only with modifications from #4087 (issue
988, commit 3741e80).

[skip ci]
Merge pull request #7583 from skirpichev/shift-issue-numbers3

Shift two issue numbers (see #7579)