Allow HorizonPlot to use TAN (gnomonic) projection

[HealthyPear]

Is your feature request related to a problem? Please describe.

For small (few degrees) regions and source-centered views I think this projection would be better
to preserve the shape of the plot (if I do e.g. az - delta, az+delta and same with zenith to get a squared plot or if I plot a circle so it shows up as a real circle)

Describe the solution you'd like
A projection keyword for HorizonPlot to change the projection wrt the default Lambert azimuthal equal-area projection.

[steveberardi]

Have you tried doing this with an OpticPlot? Those use an azimuthal equidistant projection, and although you have to specify the target in RA/DEC it'd be pretty easy to make it accept alt/az too. The target in an optic plot is also set as the center of the projection, which works well with small areas (< 9 degrees).

I'd like to keep small area plots restricted to map and optic plots, so another way we could support this is adding the Gnomonic projection to map plots and then also allowing more control of what coordinate system is plotted in a map plot (i.e. RA/DEC, alt/az, etc).

Anyway, thanks for the suggestion! I'll definitely keep this problem in mind when planning new features.