group argument without using incidence package

[ffinger]

To apply asmodee to a number of separate timeseries in a grouped dataset so far the dataset first has to be converted to a grouped incidence object. It would be useful if asmodee had a group argument to specify a grouping column when passing a dataframe.

[TimTaylor]

This makes sense and would be straightforward to implement. I won't be able to implement in the immediate future but will flag with @thibautjombart.