MTKModel: Provision to specify the type of System

[ven-k]

Type of the system can be specified via:

@mtkmodel ModelName::SystemType begin
    ...
end

By default it is ODESystem.

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

Closes #3498

[ChrisRackauckas]

Instead of this, can we just target System? Is it smart enough to handle DiscreteSystem at this point @AayushSabharwal

[ven-k]

I found only one method of System which returns ODESystem. But being able to just call it would be convenient.

[AayushSabharwal]

Is it smart enough to handle DiscreteSystem at this point

No, System always creates ODESystem. SDEs are handled in structural simplification.

[ven-k]

The test failures on Julia-Pre version are unrelated to this PR.

I've leveraged System in the macro; so once System is updated no additional changes are required to the macro.

This PR is ready for review (and to decide whether we want to add @mtkmodel ModelName::SystemType begin syntax)