[map branch] Adopt GenericMap.submap to use NDCube.crop underneath

[hayesla]

This PR is add NDCube.crop within GenericMap.submap

[hayesla]

ok, so I think this is mostly working. Two issues we need to consider:

1. How we deal with slicing to a single pixel, which can happen of course when people use SkyCoord etc where this may not be as obvious as this example. This works with the current implementation of submap in sunpy

>>> aia_map = sunpy.map.Map(sunpy.data.sample.AIA_171_IMAGE)
>>> aia_map.submap([0, 0]*u.pixel, top_right=[0, 0]*u.pixel)

ValueError: Input points causes cube to be cropped to a single pixel. This is not supported.

2. How we deal with reducing the number of dimensions. This can of course be dealt with in the submap function, just interested in peoples thoughts.

>>> aia_map.submap([0, 0]*u.pixel, top_right=[0, 2]*u.pixel)

TypeError: It is not possible to slice a map with an integer as it will reduce the number of data dimensions by one.
In order to apply the same slice without dropping a dimension do mymap[:2, 1:2].

[hayesla]

just fyi - thats why I've commented out these tests. I'll adjust when we figure it out