Bug fix: Allow different result types.

In commit 51f5c95bee3754651e30dd9f3e51c821e80221fe the `TestCase`
implementation was udpated to support any format for which a reader
exists to be used in the result section of a test case. However, the
commit did not include the necessary updates of the metamorph-test
schmema. These are added by this commit.
Minor code improvements in `PicaDecoder`.

Fixed a typo in the javadoc of the class and changed the name of a
private method to a better one.