Implement parsing of aggregated frequencies as provided by DiscovEHR

[javild]

Required for this other ticket created by @katherinesmith opencb/cellbase#315
Data can be downloaded from:
http://discovehrshare.com/downloads
Example:

#CHROM  POS     ID      REF     ALT     QUAL    FILTER  INFO
1       69081   .       G       C       .       PASS    AF=0.0130355
1       69088   .       T       C       .       PASS    AF_LT_001
1       69130   .       C       G       .       PASS    AF_LT_001
1       69149   .       T       A       .       PASS    AF_LT_001
1       69156   .       G       GC      .       PASS    AF_LT_001
1       69158   .       T       TAG     .       PASS    AF_LT_001
1       69161   .       T       C       .       PASS    AF_LT_001

AF_LT_001 tag will be stored as AF=0.005