Plane class
Non-linear system of ODEs

This PR contain the methods for nonlinear system of ODEs for 2 equations. This also includes all test cases for these methods and also test cases for classify_sysode.
Removed trailing spaces
Merge https://github.com/sympy/sympy into plane
Changed the are_perpendicular and is_parallel method
Removed a repeated condition

Condition for number of equation being equal to number of function was repeated. This commit contain removal of one of them.
Refactor code in projection_line method
Don't use the matplotlib printer if matplotlib raises an exception

Fixes #2822.
Use \left [/(/{ \right ]/)/} for latex of lists, tuples and dicts

This makes them still work with the matplotlib.mathtext renderer, which
doesn't support matrices.
XFAIL the test in test_args
Fix latex doctests
Removed the XFAIL
Added Dummies and used free_symbols() instead of is_constant()
Merge pull request #7707 from asmeurer/matplotlib_warning

Fix the matplotlib warnings
compare frozensets as Basic objects

It was wrongly being assumed that if a frozenset was encountered that
it was equal to the other frozen set. This caused arbitrary sorting
of args like Min(1,x) and Min(2,x) since they are both the same function
with a single frozenset argument...but the (1,x) should be less than the
(2,x). So rather than returning 0, the args of the frozenset(s) are
compared.
changed Dummy() to Dummy
Add tests to test_wester.py for set theory and continued fractions.

Also for symbolic continued fractions.
Removed Trailing Space error
Merge pull request #7682 from clsn/cftests

Add tests to test_wester.py for set theory and continued fractions.
Merge pull request #7667 from akshayah3/plane

Plane class