Refactor two halo

[jhod0]

Refactors the 2halo computations into a separate class, which generalizes over the 1halo model. It also very much cleans up the number of arguments to the various 2halo functions.

This addresses:

• Add full 2-halo term #1 - The two-halo term is now implemented. Some testing remains though.
• Abstract the Halo Model #2 - Partly. The 2halo term is now abstracted, but the 1halo still needs to be superclassed.
• Sort out mass definitions - mean or critical density? #4 - The 2halo class converts properly between the two mass definitions, and documents the assumptions. The BBPSProfile class also clarifies that it assumes M_{\Delta c}.

[jhod0]

Add convolution member function to TwoHaloProfile to complete #1 before merge