Warn about iteration variables without nominal attribute

[casella]

As discussed in #4395 and #4510, scaling of variables by nominal values can be critical for the performance of nonlinear solvers.

In user models it can easily happen that some badly scaled variables in nonlinear equation systems are solved for by iterative solvers, while they lack a nominal attribute for scaling. A warning is already provided if some variables are involved as iteration variables and have a zero start attribute, but there is no warning for the above-mentioned situation, which can also be critical for efficient solver convergence.

Can you please implement a similar logic that warns about iteration variables (both at initialization and during simulation) not having a nominal attribute?

This warning is only meant for debugging by advanced users, so it should be turned off by default and turned on with some debug flag, e.g. -warnNoNominal

[ptaeuber]

The warning is introduced in OMC commit ​5682249.