Makes the options to set the PSD methods. Retains the default behaviour
(will use aLIGO if nothing is set). But now, users must call a
`set_from_` method after initialising the PowerSpectalDensity()`.

Also adds logging messages

Previously, set_from_frame not only loaded in the time domain data, but
also applied a filter and FFT'd. This is no longer required and has
hence been removed.

- Fix error in create psd
- Fix minimum and maximum frequencies
- Improve documentation

Adds functions to set the strain data from open data, generate a PSD,
filter, and window.

- Adds a time_array and time_domain_strain to the
  InterferometerStrainData
- Adds a test for consistency between the time and frequency domain
  strain

- Adds a specific function to work out whether the frequency_array or
sampling frequency and duration where given
- General pep8 clean up
- No changes to functionality

Fixed a bug in create_time_series that produced incorrect time_arrays if a starting time other than 0 is given

Fixed a bug in create_time_series that produced incorrect time_arrays if a starting time other than 0 is given

Fixed a bug in create_time_series that produced incorrect time_arrays if a starting time other than 0 is given

Moves the plot_walker method to the Results object. The benefit here is
that we can generate walker plots after the run and mitigate the chances
of a plot error spoiling a run.

- Also adds ability pass through the 'a' parameter used by emcee

Make the corner plots use 1, 2, and 3-sigma countours

See merge request Monash/tupak!89

Based on http://corner.readthedocs.io/en/latest/pages/sigmas.html

Adds the emcee sampler class

See merge request Monash/tupak!88