Add prevalence of mutations to MiCall report

[wrpscott]

Micall should write out the prevalence in percentage onto the mutations.csv file, so that these can be reported, e.g. in the form 'K103N(100%)'

We should also add coverage columns to the two counts files, so it's obvious what the denominator is when calculating prevalance.

• add coverage to nuc.csv
• add coverage to amino.csv
• add prevalance column to mutations.csv
• combine mutation and prevalance on the report