Fixing some argument bugs in pymc3.py.

See merge request !299

Further fix to encapsulation functions

See merge request !297

- Fixes bug in which the arguments here not converted to floats
- Fixes typo in lamda1 -> lambda2

allow aligned spin (chi) for BBH and non-aligned spin for BNS

See merge request !292

make plot marginals work

See merge request !294

Added tutorial for fitting a line with x and y

See merge request !284

pymc3.py: explicitly set step method arguments

See merge request !293

 - PyMC3 only applies the step_kwargs and nuts_kwargs arguments
   to the step methods if they are automaticallu allocated. Otherwise,
   it assumes you have already initialised the method with the
   required keywork arguments. This was not being done in bilby, so
   this patch fixes it and does explicitly initialise the step methods
   with values passed to step_kwargs or nuts_kwargs as required.

Attempt to allow compound step methods for PyMC3

See merge request !289

allow beta distribution to have user specified loc/scale

See merge request !291

Remove outdated references in docs to BBH priors

See merge request !288

 - refs #256

Add a check that the outdir exists to plot_trace

See merge request !287

Solves a bug where run will crash if plot=True using dynesty and the
outdir had not previously been made.

Updating tutorial notebooks

See merge request !282