Enhancement: Add BMS group multiplication calculations

[duetosymmetry]

@keefemitman and I worked out what to do (requires being able to conjugate a supertranslation ɑ(θ,ɸ) by a Lorentz boost; then everything else will follow easily). The question is whether there should be a new type BMS added. I think it's natural to add such a type. However, current code stores BMS transformations as dicts rather than an object of type BMS, so to make a uniform interface, several APIs may change. Requesting comment from @moble

[keefemitman]

I think it should suffice to just define a function which takes as inputs two dictionaries of BMS transformations and returns a single dictionary of BMS transformations. That way we can easily write this function now and make sure it works with the transformation function before making things more "formal".

[moble]

Yeah, I think using dicts as the actual data structure would be best for backwards compatibility. That's not to say it wouldn't be fun to have actual classes implementing these properties underneath, but for the purposes of the user interface, inertia weighs heavily.

[keefemitman]

Can we close this? @duetosymmetry

[duetosymmetry]

Sure!