Convert parameter names in SamplesDict

The purpose of this MR is to provide a function to standardize the parameter names in the SamplesDict object. This will, e.g. map the dictionary {"mass1": [1,2,3,4], "mass2": [4,3,2,1]} to a dictionary with the "standard" PESummary parameter names, {"mass_1": [1,2,3,4], "mass_2": [4,3,2,1]}. It also allows for any custom mapping to passed.