Canonical unit representation --> CellML units issue: non-dimensional units

[nickerso]

When serialising canonical units representations to CellML units, the model compactor code will re-use equivalent units rather than having duplicate canonical units. When non-dimensional units are present in a model (e.g., cm/cm) they are equivalent to dimensionless units (e.g., TODO), and so that will get reused.

Given the purpose of the model compactor algorithm, this probably doesn't matter. But it might be nice to preserve some indication of a non-dimensional unit?