Improve the bilby_result interface

1. Deprecate the convert feature in place of a more general "save"
2. Fix the result parse in method to accept either positional or optional
3. Force the user to pick a single option using a mutually exclusive parser
4. Add an option to fix the maximum number of samples
5. Add a lightweird option to strip down the results
6. Improve the evidence printing output
7. Minor adjustments to core internals to help debugging