Add dplyr::summarize method?

[patrickbarks]

I didn't initially add a method for summarize alongside the other tidyverse functions because summarize won't ever return a valid CompadreDB object. But now I keep forgetting that I have to use CompadreData() or as_tibble() before calling summarize().

Compadre %>% 
  CompadreData() %>%
  group_by(OrganismType, Continent) %>% 
  summarize(n = n())

In hindsight, I don't think the 'must return a CompadreDB object' requirement was necessary, and a summarize method that returns a regular tibble would be rather convenient. E.g.

Compadre %>% 
  group_by(OrganismType, Continent) %>% 
  summarize(n = n())

Everyone ok with this?