Patch XML parser to protect from div-by-zero.
For specific MC generators (i.e. BcVegPy), a protection for div-by-zero is needed in the code which computes the total cross-section. For BcVegPy in particular the XML contains some cross-section values which appear not to be extracted correctly from the generator and the default (0) is stored in XML which complicates a bit more the parsing as these nodes need to be explicitly neglected during merging.