More sample CellML files

[jonc125]

Following on from #22; cf #21 :

• A private=in public=out variable
• A purely algebraic model (no ODEs)
• Error cases, e.g.
  • Simple CellML grammar errors (checking by RELAX NG schema)
  • Connecting to a hidden component
  • Inconsistent units (at connections)
  • Overriding built-in units
• There's also the collection of real, annotated, CellML models at https://github.com/Chaste/cellml for testing the parser @tamuri