BNS: ability to sample in and convert to \tilde{Lambda} and \delta\tilde{\Lambda}

At the moment we have functionality to convert lambda_1 and lambda_2 to lambda_tilde and delta_lambda (see lines 190 in conversion.py), but I don't think we have the functionality to go the other way. Moreover, I think we can only sample in lambda_1 and lambda_2 at the moment, but it would be nice to be able to directly sample in lambda_tilde and delta_lambda.

Action items (possible incomplete):

• add function to conversion.py: lamda_1_lambda_2_to_lambda_tilde_delta_lambda
• add ability to sample directly in lambda_tilde and delta_lambda
• add an example