dplyr::group_by doesn't return fhx objects

[brews]

dplyr::group_by doesn't return fhx objects. This means that users can't use burnr functions after dividing an fhx object by -- for example -- "series", because most of our burnr function explicitly check that input is an fhx object.

We should write a generic method for group_by so that it returns an fhx/tibble/whatever and our burnr functions accept its input.

Might also think of writing as_fhx() and as.fhx() functions to help users easily translate tibbles and data frames to fhx objects (see #120 ).

[brews]

As a quick fix can use the newly added as.fhx() in tidyr pipelines that use group_by().

For example, here is a (roundabout) tidyr way to get the first-year of each series in the lgr2 example data:

library(burnr)
library(tidyr)
data(lgr2)

lgr2 %>% 
  group_by(series) %>% 
  summarise(fy = summary(as.fhx(.data))$first_year)

So, we convert the grouped data back to an fhx with as.fhx(.data) and then run it through our burnr functions.