Add data_period to the bandwidth file headers
quick question about the new bandwidth file 1.4.0 headers: shouldn't data_period be in the header information? The spec defines it as "last data_period days. (data_period is 5 by default.)" but unless I'm missing something there's no way for the bandwidth file reader to know if data_period is the default or something else (and by extension what time period all these stats are over).