This reverts merge request !171

 - fix calls to likelihoods due to changes in Monash/tupak!152
 - allows user to specify multiple step methods for different
   parameters

Pymc3 works with GW likelihood (although not with the NUTS sampler as no grad function is defined in the theano Op yet)

 - complete the radioactive decay example to test the Poisson
   likelihood
 - refs Monash/tupak!132

 - this example is not finished yet!

This allows the waveform generator to be given None for the sampling
frequency and duration (in this state, it therefore can't be used to
generate an injection). Then, when passed to the likelihood, a check if
performed to see if those attributes are set. If they are None, they are
set from the data, and if they disagree with the data, an error is raised.

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