Allow time domain approximants

Closes #308 (closed)

This changes the lalsimulation function being called and enforces frequency bounds on the waveforms returned.

I noticed there's a lot of duplicate code in the source functions. I'll make a separate issue to rationalise this.

@michael.puerrer can you check this?

UPDATE: I got carried away and made all the lal source functions call a single base function. This is likely a more maintainable solution.

bilby/gw/source.py

@@ -3,7 +3,7 @@ from __future__ import division, print_function
 import numpy as np
 
 from ..core import utils
-from ..core.utils import logger, spherical_to_cartesian
+from ..core.utils import logger
 from .conversion import transform_precessing_spins
 from .utils import (lalsim_GetApproximantFromString,
                     lalsim_SimInspiralFD,