Reproject NDCubeSequence to NDCube

[DanRyanIrish]

Description

Create a method on NDCubeSequence that:

• Uses NDCube.resample (#429) reprojects each NDCube in the sequence to the WCS of the first NDCube;
• Stacks the data arrays along an additional 0th array dimension
• Generates a new WCS that adds an initial array axis to the WCS of the first cube. Let this new axis simply be in pixel units. A later issue can generate physical coordinates for this axis based on user input of cube global_coords.
• Returns a new (n+1)-D NDCube using the just the reprojected and stacked data array and the newly generated WCS object. Drop all other information to start with.