Use the bandwidth-file-spec.txt keywords as BandwidthFile attributes

BandwidthFile descriptor was implemented in #29056 (closed). Some of name of the header attributes correspond to the keywords in the bandwidth-file-spec.txt, but not all. Since we might change the keywords, the implementation might be easier to follow the keywords using the same name for the attributes. Currently these attributes are:

consensus_size
eligible_count
eligible_percent
min_count 
min_percent

Also, measurements should be named bandwidth_lines according to the specification. The bandwidth lines might not be raw measurements, but scaled.