- time_duration -> duration
- fix spelling error tiime -> time
- frequencies -> frequency_array

This reverts commit 2ab4bddd.

- Moves create_default_prior to the core prior file
- Makes fill_prior a class attribute
- Makes the BBH default priors its own file which is read in when
  figuring out the defaults
- Checks the example all still run

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