Rewrote topmost taxon graph data extraction.

Rewrote computation of topmost taxons:

1. Extract names of most abundant taxons in survey over selected time period.
2. Build top taxon dataframe by selecting only entries in macrofauna survey/timespan dataframe matching most abundant taxon names.
3. Build 'Others' dataframe by:

• Selecting all entries in macrofauna survey/timespan dataframe not matchin most abundant taxon names.
• Replacing all taxon names with 'Other'
• Grouping counts by time_group and averaging count values.

4. Concatenate top taxon dataframe with 'Others' dataframe.

assigned to @mhoebeke

mentioned in commit 49a45cd0

merged