Add identity conversion and generation functions

It is claimed in the parser for bilby_pipe that one may choose to explicitly not use any conversion function or generation function by passing "noconvert" to conversion-function and generation-function respectively.

However, this is not currently correct: at present these arguments go to this logic and this logic respectively, which both map "noconvert" to None. However, when these get passed to e.g. the waveform generator here, None is then mapped to using convert_to_lal_binary_black_hole_parameters.

This adds identity map conversion and generation functions. Conversion makes no changes to the parameters but returns the correct signature, while generation makes changes such as computing SNRs and recovering marginalized parameters, but does not attempt e.g. mass conversions.

I'll prepare a corresponding MR in bilby_pipe on Monday to handle remapping _lookups so that the correct functions would be used.

I've left this as a draft because I'm not confident that the steps with respect to e.g. SNR will work correctly for identity_map_generation, and am not sure the best way to test this robustly.