Implementation of `si_to_mtk`

[MarcusGalea]

Super cool package! I wonder if there's high demand for the following implementation,

Is your feature request related to a problem? Please describe.
We desire to include StructuralIdentifiability functionality as an intermediate part of a larger ModelingToolkit pipeline for system analysis. Specifically in order to pass MTK models into find_ioequations and convert the result to MTK models.

Describe the solution you’d like
An inverse function of mtk_to_si, i.e a function si_to_mtk for which

si_to_mtk(mtk_to_si(mtk_ODE)[1]) == mtk_ODE

generally returns true.

[pogudingleb]

@MarcusGalea , thanks for your interest in the package!

We can definitely add some interface function if there is an interest in incorporating the package into a bigger pipeline. But I would like to understand what kind of interface would be the most useful for you.
The returned value if find_ioequations is not of the structure used to define the input model but just a dictionary with values being polynomials representing (high-order nonlinear) differential equations relating inputs and outputs. Are these the relations you ultimately wamnt? If so, the function would be a bit different from just inverse of mtk_to_si.