Add full 2-halo term

[jhod0]

Add an interface to compute a projected and convolved two-halo term, just as with the one-halo term

[jhod0]

Almost complete. Still TODO: make the 2halo computations into a separate class (to easily swap in different 1halo models, as #2), and clarify the mass definitions of inputs (#4). When those are done I'll close this.

[jhod0]

This is partially addressed by #5 - we still need the convolved 2h term though

[jhod0]

Fixed by projection/convolution method added to #5