Fix for ROOT-5516 and PyCool related issues

Several improvements:
  o Selectively add classes to rootmap files according to the value of the rootmap attribute value
  o Support fully compliant xml files, e.g. with < and > expressed as &{l,r}t; respectively
  o Stop genreflex in case of errors during xml parsing

To be tested by next and forthcoming slot2 lcg nightlies.