automatically infer Ngals in mc_generate_nfw_phase_space_points

[aphearin]

The following code raises a broadcasting error even though it is clear that Ngals should be 2 and not the default argument of int(1e4):

>>> nfw = NFWPhaseSpace()
>>> posvel_table = nfw.mc_generate_nfw_phase_space_points(mass=(1e12, 1e13))