Add ability for users to check data for valid header values

[javerbukh]

This reuses code from the previous iteration of Cubeviz (https://github.com/spacetelescope/cubeviz/blob/master/cubeviz/data_factories/ifucube.py) that would allow users to check their data for valid header values. It will also output valid header values for them to use in the case that they have something invalid. This PR also includes a concept notebook that can be sent to notebook leads so that they can make sure their data is valid before using any jdaviz applications.

@rosteen If you can, can you check Tracy's original data (before you started fixing it) to make sure this would've caught all the problems the data ended up having? Thanks!