[Branch]: differential_rotation

This is the feature branch that contains some diff rot code, I think it needs some love so I am opening this so people don't forget about it!!
I don't think we even need to test if a Quantity has been passed - the code
will fall over when the wrong Quantity has been passed, or when no Quantity
has been passed.

On Tue, Nov 4, 2014 at 9:10 AM, Stuart Mumford <notifications@github.com>
wrote:

> In sunpy/image/coalignment.py:
>
> > @@ -44,6 +46,18 @@ def _default_fmap_function(data):
> >      return np.float64(data)
> >
> >
> > +def _is_pixel_unit(a):
> > +    """Tests the input to see if it is an astropy Quantity with units in
> > +    pixels."""
> > +    return (isinstance(a, u.Quantity) and a.unit == 'pix')
> > +
> > +
> > +def _is_arcsec_unit(a):
> > +    """Tests the input to see if it is an astropy Quantity with units in
> > +    arcseconds."""
> > +    return (isinstance(a, u.Quantity) and a.unit == 'arcsec')
>
> @wafels <https://github.com/wafels> rather than forcing the user to pass
> in a Quantity in arcsec we should allow any unit that can be converted to
> arcsec. We can verify that a Quantity has been passed in then do a.to
> ('arcsec') the resulting error from this is relatively useful, in that it
> says something along the lines of "Quantity can not be converted to
> Requested unit arcsec" iirc.
>
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> <https://github.com/sunpy/sunpy/pull/902/files#r19803889>.
>