Improve raw_header structure in EVTPump

The structure contains nested lists which does not make much sense, as @mlincetto reported in the chat:

raw_header from an EvtPump looks like this: defaultdict(<type 'list'>, {'propag': [['MUSIC', 'seawater', '02.03']], 'end_event': [[]], 'seabottom': [['3450.']], 'norma': [['0', '810000']], 'start_run': [['1']], 'simul': [['MUPAGE', '03.06-060918', '180917', '1347']], 'seed': [['MUPAGE-MT19937', '3', '1210375128', '0', '0']], 'cut_in': [['1.', '500000.', '-1.000000', '-0.087156']], 'can': [['0.000', '977.500', '324.600']], 'livetime': [['3.62e+03', '5']], 'physics': [['HEMAS-DPM', '07.01']]})