enable `by_group` for `run_pacta`

[jacobvjk]

depends on #109

• run_pacta() gains option to group the loan books by any variable specified in the by_group parameter
• if specified as NULL or "NULL", will return an ungrouped result, i.e. the "meta loan book"
• number of grouping variables must be 0 or 1
• data_dictionary and vignettes are updated to reflect flexible by_group instead of fixed group_id

[cjyetman]

@jacobvjk I think I got confused at some point while reviewing this, and now I don't really understand the difference between (or need for both) by_group and by_group_value. Can you explain that a bit?

[jacobvjk]

@jacobvjk I think I got confused at some point while reviewing this, and now I don't really understand the difference between (or need for both) by_group and by_group_value. Can you explain that a bit?

by_group is the column name of the variable you want to group the loan books by and is provided by the user. by_group_value is the set of values this variable can have. It is mainly used in loops over all values of by_group and is derived automatically.

If you set something like by_group = "net_zero_pledge", then by_group_values might be "yes" and "no", indicating whether or not a bank has a net zero pledge. Some calculations will produce outputs for each of the values separately.