Make Cube work with all versions of files

[albireox]

The datamodel for Cubes is stable enough that there is probably no good reason to prevent opening a cube file from any version.This should be fairly easy to do. Right now we check whether the version of the file is in the lookup table, and fail if not. If we issue a warning instead of raising an error, I think everything else would work, although we would want to add some check to inform the user that no maps/modelcubes exist for that version if they try to open one.

This is related to #221 and #285.

[havok2063]

I think this should only be available when using the filename keyword argument.

[albireox]

Yes, because I don't think we are planning on loading cube from intermediate reductions, or any DR earlier than DR15, this should only work if you specifically ask to load that file.

[havok2063]

I just meant that we shouldn't make this useable via the plateifu or mangaid arguments. I think filename can be used for any cube version outside of an MPL or DR. It might not be too hard to make Marvin usable with DR13 and 14 cubes. We can just raise an error if they try to access any DAP things via it.