Correctly check file extension in printNumberOfClasses

[FWuellhorst]

In regressiontest, we had an encoding error during printNumberOfClasses. A .mos file was detected as .mo, as the check is

                    # find .mo files
                    pos = filNam.find('.mo')

which should be

                    if filNam.endswith('.mo') and (root.find('Examples') == -1 or root.find('Validation') == -1):
                        # find classes that are not partial
                        filFulNam = os.path.join(root, filNam)
                        iMod = self._checkKey("model", filFulNam, iMod)
                        iBlo = self._checkKey("block", filFulNam, iBlo)
                        iFun = self._checkKey("function", filFulNam, iFun)

We did not check mos files as the function states it checks models.
Also, the troubleshooting would have been way easier if self._checkKey would just try except the encoded file reading and raise an appropriate error with the file name.
@mwetter do you agree? If so, I can push a fix.

[mwetter]

@FWuellhorst : This would be a good fix. Please make a PR.