keys() now returns a dict_keys object which is a view. So when the
actual keys are updated, so is this, but we do not want that behaviour.

1) If sigma is not given, then it will not be sampled over (or a warning
raised that you didn't specify a prior for sigma).
2) Comments added to explain the logic throughout
3) Tests added for the Gaussian likelihood

Hyper fix

See merge request Monash/tupak!114

Warnings cleanup

See merge request Monash/tupak!106

# Conflicts:
#	tupak/gw/conversion.py
#	tupak/gw/detector.py
#	tupak/gw/likelihood.py

Previously, the frequency_array was passed to infft, but it should
instead be the sampling frequency.

allow the detector minimum/maximum frequency to be set

See merge request Monash/tupak!109

# Conflicts:
#	tupak/core/result.py
#	tupak/gw/detector.py
#	tupak/gw/utils.py