Support for analytical spectrum with 2D spatial model

[IevgenVovk]

Closes #9

Adds new FitsMapSource source class that joins the analytical spectral model with the 2D FITS map of the surface brightness. The class may be set for a given source indicating the "fitsmap" spatial model type.

FitsMapSource assumes the sky map FITS defines the flux unit in the "BUNIT" keyword of the primary extension; if the keyword is missing it will deduce it assuming a unitary total flux from the total sky area covered by world coordinate system, defined in the header.

Usage example in the config file:

sources:
    - name: "diffuse_source"
      emission_type: "gamma"
      spatial:
          type: 'fitsmap'
          file_name: 'sky_map.fits'
      spectral:
          type: 'pwlec'
          norm: '1.0e-25 1/(cm2 s eV)'
          e0: '1 TeV'
          index: -2
          ecut: '10 TeV'

[IevgenVovk]

@mstrzys , any comments on this MR?

[mstrzys]

I cannot recall whether you replied via a different channel, but I cannot see any replies to my comments here.