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
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
Merge pull request #7407 from srjoglekar246/fielddocs

Docs for field functionality added to physics.vector