Split from issue #51

Modify method Ag3.snp_allele_frequencies(transcript, cohorts, cohorts_analysis, ...):

• Allow the cohorts parameter to be a dict (current behaviour continues to be supported) or a string. If a string, it must be the name of one of the columns in the cohorts dataframe.
• Add a cohorts_analysis parameter which is a string with default value set to latest cohorts analysis. This will be used if the cohorts parameter is given as a string.
• Other parameters left as-is.

Open questions:

• Should this enforce a minimum sample size? Or give that as an option?

Example:

df_snp_af = ag3.snp_allele_frequencies(transcript="AGAP006028-RA", cohorts="admin1_month")

• Use parameter name "cohort_analysis" throughout for consistency
• Add "min_cohort_size" parameter to the snp_allele_frequencies() and gene_cnv_frequencies() method, default value 10