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
Merge pull request #7407 from srjoglekar246/fielddocs

Docs for field functionality added to physics.vector