Several features and bug fixes for AbacusHOD and light cones

[boryanah]

Features:

• output in the HOD light cone galaxy files the RA, DEC and CZ (also Zobs and Zreal) for each galaxy (or make that an option?)
• when doing redshift space distortions on the light cone, currently it's taking the redshift of the entire halo catalog (which is more or less the median of that of all halos in it), but it would be more accurate to use the redshift_interp field (not sure if worth it and whether it would make AbacusHOD more cumbersome)
• output in all the HOD galaxy files whether they were produced using light cones or cubic box
• add scripts for applying arbitrary mangle masks to fits and dat AbacusHOD outputs and generating randoms
• add scripts for computing convergence and shear maps.

Possible bug fix:

• the particle subsampling in AbacusHOD is (I believe) resolution-dependent (also, note that currently only subsampleA particles are used; does this raise any issues with lower-mass samples?) -- need to fix that for using high and huge boxes.

Documentation:

• add documentation and reference to the abacus_lc_cat repo