Add units

MTK now supports units (via Unitful).
The standard library should support these as well:

# Parameters:
- `T`: [K] Fixed temperature boundary condition
"""
function FixedTemperature(; name, T)
    @named port = HeatPort()
    pars = @parameters T = T [unit=u"K"]
    eqs = [
        port.T ~ T,
    ]
    ODESystem(eqs, t, [], pars; systems = [port], name = name)
end

better yet, inputs should allow units:

@named fixed = FixedTemperature(T=30u"°C")

with unit translation done internally.

Best would be for MTK to deal with units (and not require inputs to be stripped)

No source file, test, or entry point is identified in the issue. First clarify whether unit handling belongs in the standard library or MTK, and define the supported input and translation behavior. Done should include standard-library support for units and a clear decision on whether inputs such as T=30u"°C" are accepted without stripping units.