New cross section parametrizations and decay methods

Features

• add ability to sample uniform phase space (4109287a)
• ability to use matrix elements in decays (4fa776d7)
• add a new leptonic decay channel (4e39af5e)
• add new bremsstrahlung parametrization (f7a1b11f)
• add new epair parametrization (67fbd900)
• add SMPs with both charges +1, -1 (2e8aa240)

Bug Fixes

• empty hardbb table for ParticleDef builder. (1ff46240)

Tests

• modify test for new/multiple epair parametrization (6d0196b7)