Symbol: change unit to preferred unit, include dimensionality

[clegaspi]

This change will clarify the unit in a symbol as a preference, not a requirement, as the requirement would be at the model level. In the docs, we will clarify that the choice of preferred unit establishes the typical magnitude of the material property, which is important for comparison of quantities of the same type to avoid floating point errors.

For unit dimensionality, we will expose pint's native "dimensionality" as a property of the symbol.

[maxhutch]

Pint's dimensionality functionality is great :-)