Add helper to reverse crossmaps

[cynthiahqy]

Previously posted in #67

Reversing Crossmaps

Crossmaps are generally one-way, but if we look at link types:

• recode links can be reversed without editing weights,
• split links can be reversed by changing the weights to one
• collapse links can't easily be reversed (since you need to specific splitting weights).

Provide a helper method to "invert" a crossmap which:

• [x] accepts an crossmap as a data argument
• [x] checks the inversion can be performed (recode, split maps only)
• [ ] swaps the col_from and col_to attributes
• [ ] adds a new weights column if necessary, and modifies the col_weights attribute accordingly
• [ ] returns a valid crossmap (recode, collapse only)

You could provide this method for any xmap subclass:

• [ ] add generic xmap_invert(xmap, ...)
• [ ] add method xmap_invert.xmap_df(xmap, .name_weights = "new_weights")

The use-cases include:

• reversing a disaggregation operation (e.g. after some adjustments have been made at the disaggregated level)
• reversing a recode (though xmap/conformr still feels a bit overkill for this use case)

[cynthiahqy]

done with #86, though might be modified depending on #67 implementation