Compute tick angles by advancing a small amount in axes coordinates

This handles cases when a plot extends over a small number of pixels in
the WCS projection, but a large distance or number of pixels in display
coordinates. Addresses #45.
spacing has to be quantity for ScalarFormatterLocator
added format_unit to change units
oops forgot to remove a * self._format_unit
added set_format_unit method
AngleFormatterLocator now takes quantities, updated CoordinateHelper accordingly
specifying units works now,merged conflict
fixed some variables change errors
cleaned up some code
removed some more spaces
small change to locator method
made small changes to clean up
added test for changing axis units,also uses smaller data cube
removing assumption that AngleFormatterLocator has angles in degrees
ticks' width and old dataset was causing image tests to fail
implemented Tom's suggestions
Add unit test
changed baseline images to have smaller size
one last cleanup
forgot to remove cache=False in one place