tab: support per month histogram

[synrg]

Support per month with or without an accompanying date range.

If per month is given, then the an observations histogram is requested for month_of_year. If a date range filter is used in conjunction with per month (e.g. since nov 2020 per month, or added since nov 2020 per month) then instead of a seasonal histogram with 12 rows, one per month of the year, counts for months since the start of the range will be shown, month-by-month since the base date until the end of the range. Other increments should be supported. See the values of interval for the GET /v1/observations/histogram endpoint for a complete list, and these notes on that field:

Time interval for histogram, with groups starting on or contained within the group value. The year, month, week, day, and hour options will set default values for d1 or created_d1 depending on the value of date_field, to limit the number of groups returned. You can override those values if you want data from a longer or shorter time span. The hour interval only works with date_field=created, and this you should filter dates with created_d[1,2]

Jared (blazeclaw) asked for this feature on iNat Discord.