Closes #68 by giving a simple interface to set the log-level (unless set
at the script level) and whether or not to use cached data.

Revert "Revert "Moritz Huebner: Removed default parameters for sampling frequency and time duration in the waveform generator""

This reverts commit 91e90f0d.

Revert "Moritz Huebner: Removed default parameters for sampling frequency and time duration in the waveform generator"

This reverts commit e4ac1430.

Moritz Huebner: Removed default parameters for sampling frequency and time duration in the waveform generator

The samplers and prior access the parameters via
`likelihood.waveform_generator.parameters`. But, this is the only way
that the waveform_generator was accessed outside of the likelihood. We
simplify this by insisting that the Likelihood has the attribute
parameters (which is set, in this case, from the waveform_generator).

- Add attribute parameters to likelihood
- Remove all look-ups of waveform_generator outside of the likelihood.
  This means the waveform_generator is a likelihood-specific thing!

If a label is given to setup_logger, the output is written to a file.

- Adds an example showing the simplest number of steps to run PE on
  GW150914
- Adds a helper (rapper) function to quickly get the BBH likelihood
  instance