First attempt at adding some support for IRIS

This is a pretty basic attempt, it adds a function that converts a
SJI cube into a MapCube, by replicating the header.

Also added a 'blank' map source for this type of image.
Add IRIS to map sources __init__
Change class doc string!
Add IRIS to changelog
Change hdu[0] to hdu for optional indexing
Modify GenericMap.rotate to allow rmatrix as argument.

Allow rotate() to accept either angle or a linear transform rotation
matrix. This is to facilitate WCS pci_j matirx rotation.
Add method to align IRISMap by rotation from header