Add test for rewrite expr using cos
Shift issue numbers in strings like issue([0-9]+), closes #7235

n -> n + 3099, see #7235

Remaining stuff:
$ grep -R 'issue[0-9]' [^.]*|grep -v '^sympy/mpmath/'|grep -v 'bugs.python.org/issue'
sympy/concrete/tests/test_sums_products.py:def test_evalf_fast_series_issue998():
sympy/integrals/tests/test_integrals.py:# issue1012

First was introduced in a945bc6
Second - in 98b4fc9
Add medium [skip ci]
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
Merge pull request #7579 from skirpichev/shift-issue-numbers2

Shift issue numbers in strings like issue([0-9]+), closes #7235
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