Add overall gene summaries

[williamdlees]

Produce statistics from published summaries:

• how often have we seen each observed gene?
• average frequency?

Allow thresholds on some stats, e.g. proportion of observed unmutated sequences.

[williamdlees]

• high/low frequences alongside mean
• links to individual alleles showing sequence and links to submissions/genotypes
• download capability

[williamdlees]

check which reference sequence to use - currently F+ORF+inframeP