Support multiple floating base bodies in multibody_utils

`MakeNameToPositionsMap` is likely due for an update, given the comments surrounding floating base bodies when it was originally written. I suspect this will have some downstream effects on logging, so will have to be done with care. @AlpAydinoglu hit this issue when trying to use multiple floating bases.

https://github.com/DAIRLab/dairlib/blob/master/multibody/multibody_utils.cc#L182