Allow for different metrics for different models in `mopyregtest.Generator`

[pstelzig]

Problem description

Right now, all tests generated from a mopyregtest.Generator object use the same metric for all tests it generates. However, it could make sense to use different metrics for different library elements. Right now, this can only be achieved by creating regression test definitions manually, or use different mopyregtest.Generator objects (each of which generates its own test definition file).

Desired behavior

In mopyregtest.Generator have to option to use dedicated comparison metrics for individual Modelica models, and use a default metric for all others.

Potential solution

Use the metric passed to the construtor of mopyregtest.Generator as the default metric, and pass dedicated metrics in the mopyregtest.Generator.generate_tests method, just like with the pre-existing references as a dictionary mapping model name to metric to be used.