Integrate halo model for intrinsic alignments in 3x2-point systematics.

[chrgeorgiou]

This PR integrates the halo model for intrinsic alignments as a potential systematic for 3x2-point likelihoods. It adds a new WeakLensingSystematic class called HMAlignmentSystematic and the appropriate behaviour for when a tracer has this systematic attached to it. It also extends the ModelingTools for computing halo model-related quantities. Finally, it computes the correct power spectrum given the input tracer.