Add units to component parameters

[bauglir]

Many components have nice descriptions for parameters and include the corresponding unit information in their comments, e.g. the Resistor or the ConstantPermeance. However, even though this appears to be supported by the @mtkmodel macro, the unit of these parameters is not defined. It would be nice if this additional metadata was made available within the component definitions for introspection.

I'm not sure if the same would be possible for states, but if it is then I imagine that'd be useful as well.

[bauglir]

Seems related to #90 (this one is newer and hence the duplicate).

[ven-k]

#214 is switching to use DynamicQuantities. We need ability to define custom units (like Wb). The foll. PR adds that:

• Allow External Unit Registration SymbolicML/DynamicQuantities.jl#107

[ven-k]

Related:

• feat: accept additional metadata in diff2term JuliaSymbolics/Symbolics.jl#1104
• fix: pass VariableUnit of diff var while converting it to a Term ModelingToolkit.jl#2583