Allow any value in the "period_by" column in *_allele_frequencies_advanced()

[leehart]

Allow any value in the "period_by" column to allow for more flexible temporal grouping, particularly useful if want a span of multiple years.

[leehart]

Re: comment/question #694

[alimanfoo]

Hi @leehart. I think what we're looking for is an option to provide the name of a column as the value of the period_by parameter.

I.e., if period_by is one of "year", "quarter" or "month" then the behaviour is unchanged. If it is any other value, then assume it is the name of a sample metadata column, and group on that column as-is (i.e., assuming the column already contains something sensible that can be grouped on, like instances of pd.Period).