Added abstract class BioModelWriter to semsim.writing. All writers are
now subclasses. Removed Writer.
Streamlined the Encoder class and refactored it to reflect the above
changes.
Added CellMLUnits.txt to /cfg and moved all CellML units from
CellMLConstants to the file. It is now loaded and accessed via
SemSimLibrary.
Moved annotation package from model package to the same level.
Added variables and getters and setters to PublishingMetadata.