Adds ValueError to mapbase.submap() when Helioprojection coordinate contains NaN values.

[grahamasam]

This pull request produces a value error when Helioprojection coordinates that have NaN distance values are input as bottom_left or top_right to the submap() function.

Fixes #5598

Additional Info:
I'm unsure if this solution fully addresses the common user issue presented in #5598. A testcase that mimics one of these specific user issues could be useful to determine what other situations should result in the ValueError. Any suggestions would be greatly appreciated!

[wtbarnes]

If we merge #7115, this should change to SphericalScreen

[wtbarnes]

This reads more like a warning than an exception. My issue with it is it doesn't really say why an exception is being raised. Why am I not being allowed to crop my map when one of my coordinates is off disk?

[nabobalis]

Why am I not allowed?

[nabobalis]

I added more text to the message, see if you like it.

[wtbarnes]

Same as below. Should be updated to use SphericalScreen once that PR is merged.

[nabobalis]

TODO:

• Rebase this PR when Provide a planar screen as an optional assumption for handling off-disk helioprojective coordinates #7115 is merged in

[ayshih]

Oops, I guess I haven't looked at this PR before. As guessed by the author, this PR doesn't actually address the common use case of #5598 because the implementation is at the "wrong" place in the code. (The included unit tests do not reflect common usage.) I'll try to provide better guidance soon if someone else doesn't do so first.

[ayshih]

Here's an example of the common use case:

>>> import astropy.units as u
>>> from astropy.coordinates import SkyCoord

>>> import sunpy.map
>>> from sunpy.data.sample import AIA_171_IMAGE

>>> aiamap = sunpy.map.Map(AIA_171_IMAGE)

The following submap that includes off-disk pixels works fine because it is in the native coordinate frame, so 2D coordinates do not need to be promoted:

>>> coord_native = SkyCoord(0*u.arcsec, 0*u.arcsec, frame=aiamap.coordinate_frame)
>>> aiamap.submap(coord_native, width=1000*u.arcsec, height=1000*u.arcsec)

The following, very similar submap fails because the observer is slightly different, so 2D coordinates need to be transformed to the native coordinate frame, and thus the off-disk 2D corners get a NaN distance when internally promoted.

>>> coord_other = SkyCoord(0*u.arcsec, 0*u.arcsec, frame='helioprojective', observer='earth', obstime=aiamap.date)
>>> aiamap.submap(coord_other, width=1000*u.arcsec, height=1000*u.arcsec)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
...
ValueError: cannot convert float NaN to integer

The check needs to be on the output of self._parse_submap_input() right here:

sunpy/sunpy/map/mapbase.py

Lines 1912 to 1913 in e7536b1

	pixel_corners = u.Quantity(self._parse_submap_input(
	bottom_left, top_right, width, height)).T