Initial versions of objects, barely working.
Various minor changes and optimizations.
Update docstrings and comments.  Clean up some code.
Add argument info to urqmd.tostdout() docstring.
Add event reader.
Use class method to calculate flows.
Minor optimization.
Update docstrings.
Minor simplification.
Add particle filter class.
Update docstrings.
Temporarily disable Events/Batches in UrQMD.
Clarify UrQMD particle dictionary.
Convert UrQMD class to a generator [simpler + faster].
Remove defined MCID function:  it was only called once.  Also marginally faster.
Simplify particle IDs variable name.
Ignore particle/antiparticle when filtering IDs.
Replace old event reader.  Add particle filtering.
Fix typo.
Move PDG functionality to its own class.