Add metadata functionality

[cpfaff]

Fetch (meta-data) the data structure from BExIS to provide this information to
the user maybe as attributes attached to the native data object in R. BEXIS
supports a host of different metadata schemata. We cannot support all of them
for the import into R. Thus we better go with the conversion mechanism also
provided from BEXIS which allows to convert and return metadata in one specific
format. It would be good to go with the Ecological Metadata Language as there
is already an R package that supports handling the format.