Break up `utils` into subpackages with better names

[maxnoe]

Something called utils will mostly end up as a catch it all module of unrelated stuff.

[kosack]

current contents (which you can see are already pretty unrelated):

• astro: star catalog access
• datasets: access to external (supporting) data like ctapipe-extra or files in the CTAPIPE_SVC_PATH
• CutFlow: class to apply and track cuts applied to events
• fitshistogram: class to create and manage N-D histograms, with read/write to FITS
• json2fits: convert JSON docs to FITS headers (not used?)
• linalg: linear algebra helper functions
• dynamic_class: construct a class by name (I think). Doesn't seem to be used anymore...
• table_interpolator, template_network_interpolator, unstructured_interpolator: general ND interpolators used by ImPACT (and later perhaps other things like IRF interpolation)
• rgbtohex: color conversion for e.g. web guis using Bokeh

Things that I think could belong together:

• astro + datasets (all correspond to data/table access) → ctapipe.datasets or ctapipe.service?
• fitshistogram + all the interpolators (relate to interpolation and histogramming) → ctapipe.cube perhaps?
• linalg → ctapipe.reco (used so far only in there I think)
• json2fits → ctapipe.io or simply remove if not used (I think it was for serializing config files to FITS)
• rgbtohex → ctapipe.visualization or ctapipe.plotting
• cutflow: not sure
• dynamic_class : remove?

[maxnoe]

I think it was for serializing config files to FITS)

God help us

[maxnoe]

Had a look at linalg:

• rotation_matrix_2d: should go into the coordinate trafos
• length: not used
• normalize and angle: verbatim copies of the same functions in HillasReconstructor and not used anywhere else

I think this module should be removed and the function defined where they are used.