Support array indexing by enumerations in the NF

[Francesco Casella]

The latest ​commit to the PhysioLibrary caused ​some regressions, see, e.g., ​Physiolibrary.Fluid.Examples.AirWaterSaturation due to the use of enumerations as indeces for arrays, see, e.g., ​line 5810.

I understand this is allowed, see ​Section 10.5.1 and it's a feature that's been around at least since Modelica 3.4.

@perost, how hard it is to implement?

[Per Östlund]

Enumeration indices are already supported, but the specification says:

Arrays can be indexed using values of enumeration types or the Boolean type, not only by Integer. The type of the index should correspond to the type used for declaring the dimension of the array.

This is not the case here, the array in question has an Integer dimension but is indexed with an enumeration expression. So the enumeration expression would need to be converted to an Integer in this case:

upperRespiratoryTract.substances[Integer(Air.S.H2O)]