Previous pull request + OpenModelica regression tests

This pull request comprises the previous one.  I can probably make another one if you have already merged that pull request.

The new changes add OpenModelica tests in two different ways:
1) when .mos files are present, the regression tests can be run as with dymola.  Specify the executable='omc' in the initialization of the Tester().  However, the models specified by a corresponding .mos file are only checked, no simulation is performed.  This can be added later on.
2) on any library, all example or test models can be detected based on their subpackage name and compilation can be checked.  This is now very similar for JModelica and OpenModelica:

```
t=regressiontest.Tester()
t.testOM(...) # run OpenModelica tests
t.testJM(...) # run JModelica tests
```