v9.13.0

ModelingToolkit v9.13.0

Diff since v9.12.2

Merged pull requests:

• add sampletime operator (#2690) (@baggepinnen)
• refactor: remove check_length in the error message of unbalanced system (#2693) (@sathvikbhagavan)
• Fix method redefinition (#2698) (@YingboMa)
• Generalize substitute_sample_time so that other backends and extend it (#2708) (@YingboMa)

Closed issues:

• Model linearization fails when no parameter is used (#2400)
• Variables eliminated in structural_simplify are not included in simulation plots (#2678)
• Add sampletime() and clock() operators (#2687)
• Silent and wrong derivatives of observed variables (#2697)
• Implicit variable erroneously required in initial condition (#2703)
• structural_simplify uses metadata of the variables instead of what is passed into the system (#2706)