Allow passing file objects to converter functions

[Hespe]

The converter functions, e.g. Segment.from_elegant, Segment.from_bmad, ParticleBeam.from_astra, currently take a str argument to determine the data source. This might be limiting in certain situation if the object to convert is already loaded into memory.

Instead, one could allow the input to be given as something like Union[str, PathLike, IOBase], allowing to either pass a path or an already opened file object. This matches the behaviour of Pandas.