* moved parsing of graphs from DataSet to DataSetConfig
* parse method now takes several datasets, which might be combined in
order to parse a graph
* DataSetConfig.parse method returns a list of graphs 
* files defined in DataSetConfig have to specify a groupName (which
corresponds to the graph the file belongs to)
* DataSetFormat can implement the interface Parsable now. Only Parsable
formats can be parsed, only the parser of such a format is directly
invoked to parse input files, optional files can be specified using the
Parsable.optionalInputs method 
* TODO: implement