Source file parser not working with arbitrary mfd and None bin_width

[klunk386]

When trying to parse a multisource file with arbitrary mfd (discrete) using:

conv = SourceConverter()
group = nrml.parse(xml_file, conv)

the parser fails if the bin_width parameter is None: ValueError: node bin_width: Could not convert bin_width->positivefloats: could not convert string to float: None, line 41 of Data/GridMultiSources_S10_Inc_Mod.xml

I guess this parameter should not be mandatory for this type of mfd. If so, however, then the bug is within the write_source_model, which does not perform a check or use a default value.

[micheles]

I need the file to be able to debug the issue