MNT: Move costools to here

[pllim]

As brought up by @stscicrawford and discussed with @stscirij , given that calcos is already in Python, there is no reason to keep costools as a separate package. Therefore, it might be feasible to move part (if not all) of costools to calcos. This way, when a change happens in calcos that would affect costools, you don't need to worry about keeping two different package in sync.

[stscicrawford]

Or the opposite of moving this into costools. It would be good to know if the COS team has a preference.

[Rplesha]

It would be good discuss this among the team before making any final decisions on this as a lot of documentation would need to be updated. However, calcos into costools makes more sense overall because costools contains calibration tools whereas calcos is the pipeline.