Use or remove zombie classes

[teutoburg]

The class PoorMansHeader appears to be used only in MonochromeTraceCurve, which itself is never used anywhere except in unit tests for itself.

@astronomyk what was the original intention of MonochromeTraceCurve? Is it a zombie from SimCADO (or SpecCADO or whatever) days? Do we have any use for it? If not, I suggest removing it.

In the case of PoorMansHeader, there might be other points to consider, see the attached sub-issue.

[teutoburg]

Same goes for spectrograph.py, which is an empty module. The existence of it has recently also caused confusion with at least one user that I know of.

Same also for fov_utils.make_flux_table(), fov_utils.combine_table_fields(), fov_utils.combine_imagehdu_fields() (except in a comment somewhere), fov_utils.sky2fp(), fov_utils.extract_common_field(), fov_utils.make_cube_from_table(), all of which seem to be completely unused except in their own unittests. Find out if there's any use case for these functions (perhaps in the context of something like MOS or the issue of "bright source just outside the FOV" etc.) or whether they're truly redundant and their functionality is done by other code, in which case they should be removed.

This also applies to all functions in fov_manager_utils.py!