Proper handling of Scalar Pixel types in itkImageRepresenter. Fixes #47.
Adapted representer tests to work with the standard representers.

The representer testcases had so far not been adapted to the new standard representers.
This commits fixes these tests and the bugs that were found by running the tests.
Merge pull request #48 from statismo/representer_tests_and_bugfixes

Representer tests and bugfixes (Fixes #47)