Set the default to None - if the ratio does not produce nans then it is
by default used.

- Add doc file on CBC PE
- Add introduction text

allow aligned spin with a<0

See merge request Monash/tupak!48

Moritz Huebner: Introduced a __repr__format_helper method to help automating __repr__ formatting for all subclasses.

Makes the decoding less aggressive. This was causing various things to
be coerced into a list when they where dictionaries etc.

- Reduces the length of the output
- Explicitly states if it is a logz ratio or just logz

If lalsim is not installed just add a warning rather than raising an
error. Previously this did already exist in source.py, but we now also
import lalsim in conversion.py as well