add ClockChange operator

[baggepinnen]

This PR adds a discrete-time operator ClockChange that changes the clock of a discrete-time variable. This allows for sub and super sampling.

Status: In the simple test model added to the tests, I get an error from the compiler

julia> @mtkbuild cc = CC()
ERROR: Some specified inputs were not found in system. The following variables were not found Any[ModelingToolkit.ClockChange(Clock(t, 2.0), Clock(t, 1.0))(y(t))]
Stacktrace:
  [1] error(::String, ::Base.KeySet{Any, Dict{Any, Bool}})
    @ Base ./error.jl:44
  [2] markio!(state::TearingState{ODESystem}, orig_inputs::Set{Any}, inputs::Vector{Any}, outputs::Vector{Any}; check::Bool)
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/systems/abstractsystem.jl:2038
  [3] markio!
    @ ~/.julia/dev/ModelingToolkit/src/systems/abstractsystem.jl:2009 [inlined]
  [4] _structural_simplify!(state::TearingState{…}, io::Tuple{…}; simplify::Bool, check_consistency::Bool, fully_determined::Bool, warn_initialize_determined::Bool, dummy_derivative::Bool, kwargs::@Kwargs{})
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/systems/systemstructure.jl:680

The problem appears realted to the hacky forward shift employed by the compiler, since

fullvars[end]
ModelingToolkit.ClockChange(Clock(t, 2.0), Clock(t, 1.0))(Shift(t, 2)(y(t)))

while the operator variable is not shifted

collect(keys(inputset))[]
ModelingToolkit.ClockChange(Clock(t, 2.0), Clock(t, 1.0))(y(t))

---

EDIT: I pushed a commit that applies the shift to the new operator as well. The remaining problem is that x is updated before y, while it should be the opposite, the equation for x depends on y and has to come after.

[baggepinnen]

@aayush Sabharwal @yingbo Ma
It looks like we need to take some time to clean up the current implementation that forward-shifts discrete time variables, it leads to headaches like above.

I'd be in favor of getting rid of the forward-shift hack completely, are there any fundamental reasons left that prevents us from doing that?