Add interpolation routines
Tidied up files
Implemented celestial interpolation from n-d slices
Check that input and output WCS are equivalent (same number and order of axes)
Added ``Returns`` sections to docstrings
Added headers and __future__ imports
Fixed newline
Implemented suggestions by @keflavich
Remove unused variable
Merge pull request #31 from astrofrog/add-interpolation

Add interpolation routines