Gnomonic projection: celestial coordinates in
red 
native coordinates in grey

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Native spherical coordinates are not often of primary interest.

Need to define a spherical coordinate rotation from native
coordinates to the celestial spherical
coordinates of interest.

Final step in the algorithm chain:

Spherical rotation specified by three Euler angles.

For historic & practical reasons, these Euler angles are defined in
a way which relates them to the image  indirectly via
CRVALi.

Firstorder approximation holds at the reference point (for most
projections, default LONPOLE):

Third Euler angle must be specified  need new keyword(s).

Additional FITS keywords:
LONPOLE, LATPOLE.
