Extend summary needed

[damianooldoni]

We thought to not extend the summary of observations with groups with zero values (or NA for n_scientificName), similar as done in dplyr.
However, these groups are still important for visualization purposes as users want to see those in the map by default.

I decided therefore to add argument extend to summarize_observations() with default = FALSE (no extensions) in order to have compact summaries by default. This feature is implemented by internal function extend_summary().

To be done:

• write function
• write documentation
• add unit-tests to test-summarize_observations.R

[damianooldoni]

Implemented in branch map_summary as this issue popped up during the development/update of the visualization functionality.

[damianooldoni]

Solved in PR #373.