Update the posterior samples writing method
This fixes some bugs in the method to write samples in a way that is L.I. like.
- Improves documentation
- Adds more methods to write filenames/outdir/label etc
- Fixes complex numbers to be abs
- Removes strings (i.e. waveform_approximants).
Ultimately, this allows one to do np.genfromtxt(filename, names=True)
.