Add function for determining the abundance in groups

For example, this can be used to determine the total abundance
within a species, or within each site in each year. Accomodates
both columns of abundance to be summed over and situations where
each row represents the sampling of a single individual.
Make order of arguments consistent for richness & abundance in groups

Making the abundance column optional in abundance_in_group required
that it come last. This commit flips moves the species ID columns
in richness_in_group to the last position for consistency.