What to do with PoorMansHeader?

[teutoburg]

This class is currently only used in the most-likely-to-be-removed (see parent issue) MonochromeTraceCurve and in some test cases. However, there is an old TODO-item (I'm seeing that TODO list for the first time...) that mentions the following:

• Remove dependency on astropy HDU objects
  WHY: They are slow. Too many internal checks on the header keywords. Factor of 10x speed increase when using ScopeSim's interal "PoorMansHeader" object
  WHAT: Expand PoorMansHeader to cover all use cases, add minimal units checks, replace all occurences of astropy HDU objects

I'm now wondering whether this is actually (still) relevant. I think in most cases the performance impact of the astropy HDU header is minimal, and it does offer some interoperability advantages. However, I haven't actually done any profiling on this to say with any certainty whether this has a relevance in practice. If it indeed does, I'm open to the idea of reviving the PoorMansHeader and using it more internally. If not, I suggest removing the class and just sticking with astropy's solution.