Add yaml support

[rkrug]

Adding yams support is easy, as a dmdSchemeSet can beconverted directly to a yaml character using the as.yaml() function in as_yaml branch.

IMPORTANT: The export and import has to follow the same logic as the xml, i.e. the round trip needs to be

• lossless if exported with the complete option
• lossless if exported with the metadata option and the scheme definition is available
• not possible when exported with the metadata option and no scheme definition is available

This means, that the definitions also need to be exported when output = complete

• add generic function and methods for as_yaml() which returns a yaml character and add class yaml_document.
• add method for yaml_document for as_dmdScheme and as_xml
• add write_yaml() and read_yaml() functions
• add all tests