Fill TDataMember attribute map starting from an XML selection file.

The mechanism followed is the previous one:
1) Add metadata to the selection rule (of the class or of the field)
2) Add it to the ast in rootcling in form of attribute
3) Read it in TMetaUtils and fill the proper TDictAttributeMap
Adopt coding convention for include guards (thanks Fons)

and profit to add some documentation in the code.