registered function unit fix

[isaacsas]

Closes SciML/Catalyst.jl#1035

In recent MTK versions registered functions have non-unit coefficients when using get_units, i.e.

mm(X, v, K) = v * X / (X + K)
mm2(X, v, K) = v * X / (X + K)
Symbolics.@register_symbolic mm2(X, v, K)
@parameters t [unit=u"s"] K [unit=u"mol/m^3"] v [unit=u"(m^6)/(s*mol^2)"]
@variables X(t) [unit=u"mol/m^3"]
mmunits = MT.get_unit(mm(X, v, K))         # 1.0 m⁶ s⁻¹ mol⁻²
mm2units = MT.get_unit(mm2(X, v, K))     # 0.5 m⁶ s⁻¹ mol⁻²

This seems to fix the issue.

[isaacsas]

@ChrisRackauckas should be good to go if tests pass.

[isaacsas]

@ChrisRackauckas can we get a bugfix release now so that Catalyst CI passes again? I'd like to run it through and then release a Catalyst version that is Symbolics 6 compatible.