Fix typo
Merge pull request #2443 from shikil/typo

Fix typo
Added code for coordinate variables handling
Added tests, changed express function
Rectified typos
Added cache for dcm
Rectified errors, added docs for CoordinateSym
Rectified ReferenceFrame doctest
Changed dcm()
Removed recursion in dcm()
PEP8 changes, added equality conditions for CoordinateSym and tests
Added tests for variable_map, resolved errors