Implement initial differential_rotation routines.

This is a major rebase and squash, below is a list of all the old
commits.
adding in rot_xy

implementing rot_xy.pro

sun_pos.py nearly done, on the way to rot_xy.py

sun_pos.py working

filling in more of pb0r

working on pb0r function

fixed error in sun_pos; error in pb0r to do with arctan?

fixed pb0r - now gives results the same as the IDL version down to the 7th decimal place approximately.

trying to track down the bug in rot_xy.py - maybe one of the WCS files?

seemingly fixed rot_xy; results now very similar to IDL's rot_xy, but still a little different.

convert_hcc_hg appears to be the correct inverse of convert_hg_hcc; in IDL, arcsec2hel appears to be the inverse of hel2arcmin.  However, the values calculated by convert_hcc_hg and arcsec2hel are different.

added some notes

completing rot_hcc hopefully, and putting that functionality into Mapcube

tidying up white spaces

tidied up rot_hcc calling, and added a spacecraft option

changing the rotation function

fixed the solar rotation function rot_hpc

minor changes

rename p0br

attempted fixes to post-staging merge

remove prints

wcs modifications

more small tweaks to util

small changes

Fix mapcube regression

Fix diff_rot tests by using almost equal

Fix diff_rot tests by using almost equal

Tweaks and 80char lines

Fix a kwarg bug and change some test values!

revert mapcube after rebase.

revert wcs after rebase

move the test file into the new location