ENH: add identity_map_conversion as the noconvert default

This maps to the MR !1264 in the main bilby library. The help statements for bilby_pipe currently claim that passing "noconvert" will result in no conversion function being applied, which is not presently True. This function aligns behavior with the help statements.

A previous branch implementing these same changes was at !610 (closed) but I have managed to do something astonishingly awful to the git history of that branch, so I have recreated it with the cherry-picked changes in this branch.