Initialize particle beam using Twiss parameters

It would be convenient to be able to initialize a particle beam in WarpX by specifying its Twiss parameters.  An example of this can be found in Wake-T https://github.com/AngelFP/Wake-T/blob/09239828d0d0f7cd2a750ddf5e0945efc796d11c/wake_t/utilities/bunch_generation.py#L15