Use MTK's default `t` and `D` throughout repository

[TorkelE]

In v9, MTK creates default time and differential variables. This PRs make Catalyst use these (throughout tests and documentation). I.e. instead of

@variables t
D = Differential(t)

we now do

import Catalyst: t_nounits as t, D_nounits as D

CI still cannot work due to all_dimensionless issue mentioned in https://github.com/SciML/Catalyst.jl/pull/785 (which this one depends on).

[isaacsas]

Can you change the base branch for all these PRs you are opening to v14 so they are readable?

[TorkelE]

The other ones are to the v14 branch already, changed this one as well now. This one is based on https://github.com/SciML/Catalyst.jl/pull/785 (because Catalyst cannot, currently, compile on ModelingToolkit#master without that one)

[TorkelE]

The rebase must have messed up, I will try to figure out and fix.