Added files and docs for primary field functionality
Merge branch 'master' of https://github.com/sympy/sympy into fielddocs
Added documentation for fieldfunctions, rectified errors
Made changes suggested by @smichr
Merge branch 'master' of https://github.com/sympy/sympy into fielddocs
Correct nth-order linear ode constants

- use a complex-conjugate accounting to avoid adding extra constants
- test nth-order constants without simplification
- remove XFAIL, now that we fix #6247
- remove XFAIL, now that we also fix #7081
Fix for Issue 7140: plot.append
modify plot.append docstring

Also, make extend more precise in terms of what it accepts.
Improve XFAIL's for test_ode

- add XFAIL test for atan2 in im(_)'s
- add XFAIL bad treatment of exp() by collect.
- fix XFAIL in test_ode related to constantsimp
Rename subset() to is_subset() and add is_superset() #7460

The subset() method is now marked as deprecated. The is_subset() method
also has a different semantic meaning now.

Earlier, self.subset(other) checked if 'other' is a subset of
'self'. However, now with the renaming it makes more sense that the
method call self.is_subset(other) is interpreted as check if 'self' is a
subset of 'other'.

A corresponding is_superset() method has been added to allow checking if
one set is a superset of another.
Merge pull request #7346 from amitsaha/is_subset

Rename subset() to is_subset() #7460
Re-enable term sorting in odesimp()
cse_main: Check for used symbols, workaround for #5331
Add test
Fix cse with iterator

failing test_multiple_expressions
symbols are lookuped up for reduced_exprs.
Cleanup whitespace
Merge pull request #7407 from srjoglekar246/fielddocs

Docs for field functionality added to physics.vector
replace Wilds in odesimp() with powsimp()
Simplified test_name_conflict
Add a function to filter out excluded symbols and test.