fix: pass VariableUnit of diff var while converting it to a Term

[ven-k]

Needs:

• feat: accept additional metadata in diff2term JuliaSymbolics/Symbolics.jl#1104.

Units are defined only in MTK, so pass this piece of info to Symbolics.diff2term to add this to the returning term-var.

With this PR: k_t's unit will be 1.0 s⁻¹

using ModelingToolkit
using ModelingToolkit: t, D, get_unit, value, VariableUnit, default_toterm
@variables k(t)
get_unit(default_toterm(D(k))) # returns 1.0 s⁻¹

And prints a clear warning message whenever the term arg has a non SI unit.

julia> @variables l(t) [unit = u"mg"]
julia> l2 = value(D(D(l)))
julia> default_toterm(ModelingToolkit.value(l2))       
┌ Warning: Ignoring the unit while converting `Differential(t)(Differential(t)(l(t)))` to a term.
│ 1.0e-6 kg uses non SI unit. Please use SI unit only. 
└ @ ModelingToolkit C:\Users\Asus\.julia\dev\ModelingToolkit.jl\src\systems\unit_check.jl:176
lˍtt(t)

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

[ven-k]

The newly added tests in "Variable Utils Tests" have passed here

As Clock calls default_toterm and the behavior shouldn't change "Parameter Dependency Test" are related too. It has passed here

This PR is ready for review.

Additionally, it removes this accidentally added file mo.diff: 27e2699