Make default value units consistent

[contradict]

When using units in a component, it would be nice if those units could be checked on the default and initial values. With old ModelingToolkit, it was possible to do @parameters p = ustrip(expected_unit, provided_value), [unit=expected_unit], but that is not longer possible. This PR is a draft of a way to restore that functionality. It only works with Unitful and only with scalar variables, but these could both be fixed if this is desirable.

The result is a component defined like this

julia> @mtkmodel ATest begin
       @parameters begin
       some=1.0, [description="Some", unit=u"kg"]
       more, [description="more", unit=u"L"]
       end
       end
ModelingToolkit.Model{typeof(__ATest__), Dict{Symbol, Any}}(__ATest__, Dict{Symbol, Any}(:kwargs => Dict{Symbol, Dict}(:some => Dict{Symbol, Any}(:value => 1.0, :type => Real), :more => Dict{Symbol, Union{Nothing, DataType}}(:value => nothing, :type => Real)), :independent_variable => t, :parameters => Dict{Symbol, Dict{Symbol, Any}}(:some => Dict(:default => 1.0, :unit => :(u"kg"), :type => Real, :description => "Some"), :more => Dict(:unit => :(u"L"), :type => Real, :description => "more"))), false)

can be instantiated like this

julia> ATest(;some=3.0u"g", name=:t)
Model t with 0 equations
Unknowns (0):
Parameters (2):
  some [defaults to 0.003]: Some
  more: more

And the units are converted correctly. Errors are given for incompatible or missing units.

Is this a desirable feature? If this PR were fleshed out to handle DynamicQuantities and array variables as well, is there a chance it could be merged?

[ChrisRackauckas]

I think it's desirable. Handling DynamicQuantities is essential.

[contradict]

@ChrisRackauckas I think this is ready for a review.