Fault occurrence rate calculators - 1st commit

Implements basic calculators for occurrence rates from geological data:

1) Anderson & Luco (1983) - "Arbitrary" Types 1, 2, and 3
2) Anderson & Luco (1983) - "Mmax area" Types 1, 2 and 3
3) Characteristic - Simple calculator for a characteristic earthquake with/without truncated Gaussian uncertainty
4) Youngs & Coppersmith (1985) - "Exponential" and "Characteristic" forms ("characteristic" implements the openquake.hazardlib.mfd.youngs_coppersmith.YoungsCoppersmith1985MFD.from_total_moment_rate function)

Tests for calculation accuracy include re-production of results found in Anderson & Luco (1983) and Bungum (2007).