remove precomp

This finally gets rid of the whole precompIFO function.  It does this
by breaking up precomp into various smaller functions, moving them
into the common IFO noise definition module, and using them
appropriately when needed.  This does result in some of the functions
being called multiple times in the full budget calculation for the
aLIGO-like budgets, but the cost should be minor given the convenience
of getting rid of the whole precomp thing, thereby opening up
everything to non-aLIGO-like configurations.

Even though some functions are calculated multiple times, we're still
orders of magnitude faster than matgwinc, so...  Should still revisit
down the line though.

closes #40