Abstract the Halo Model

[jhod0]

Make all pressure profiles subclasses of a PressureProfile (or similar) class, which will abstract projection, convolution, and the 2-halo term computation. This will make it easy to plug in alternative models; e.g. the modified profiles in Hill et al.

[jhod0]

Issue #1 should be complete before this one

[jhod0]

As mentioned in #5 - the 2halo is now both separate and abstracted, but the 1halo term still needs this.

[jhod0]

1halo still needs to be abstracted, on my TODO list

[jhod0]

branch abstract-1halo at commit b0c4d7c is working on this