New Coalignment API

[Deus1704]

Fixes #83
Fixes #103

Following our discussions, in this pr I've laid the skeleton for the new coalignment interface.
A basic use case is demonstrated, where the match_template method is registered by default. For details on the registration process, please refer to the code.

aia_map1 = sunpy.map.Map(sunpy.data.sample.AIA_193_CUTOUT01_IMAGE)
aia_map2 = sunpy.map.Map(sunpy.data.sample.AIA_193_CUTOUT03_IMAGE)
### Creating a template from aia_map1
bottom_left = SkyCoord(600 * u.arcsec, -500 * u.arcsec, frame=aia_map1.coordinate_frame)
top_right = SkyCoord(800 * u.arcsec, -200 * u.arcsec, frame=aia_map1.coordinate_frame)
submap = aia_map1.submap(bottom_left, top_right=top_right)

coaligned_map = coalignment(aia_map2, submap,"match_template")

Tasks:

• Handle clipping
• Deprecate existing API
• Add tests
• Add tests for checking the newly created functions
• Add gallery examples for the new api
• Write an exhaustive changelog
• Decide on a better name for this method than "match_template"