SBML L3V1 support

[bgoli]

Currently PySCeS imports/exports L2V4 and we need to deal with the model syntax (no defaults etc) as well as new/changes elements (event handling, math, packages, annotation).

[jmrohwer]

Do you have any estimate of the amount of work involved in this? Can one be selective about this? E.g. do we have to deal with annotations? AFAICT they are in any case not part of the spec but used by tools for their own tool-specific stuff. Were you thinking of incorporating them into the PSC file in one way or another?

I was under the impression that we have dealt with events and math, amongst others in #53 and #46.

As regards packages, which packages should we support? Maybe start with core? See also comments here: https://github.com/PySCeS/pysces/issues/51#issuecomment-900359130

[bgoli]

This is potentially some work

• support for MIRIAM annotations and SBO is pretty much standard, so needs to be done at some point (I would store them in an auxiliary JSON/XML file and add support for loading, saving COMBINE archives that contain both files).
• L3 changes the event handling a bit by introducing a priority ranking for triggering simultaneous events.
• there are a few expanded symbols but Core features like InitialAssignments and the removal of all implicit defaults will need to be checked and tested (ideally I would like to rewrite the whole math subsystem to get rid of the infix evaluation but this is a big step so not for the near future, and only if it is faster)
• Distrib and Comp would be the first I would look at adding some level of support for, but this is optional