Perform metadata<->data synchronisation checks in `modify_data(entity)` before returning result

[bobular]

If data columns have appeared or disappeared, it can recommend running sync_variable_metadata()

Consider providing a column renaming function that would take care of renaming the variable metadata row (variable column) as well. This would make sure that metadata doesn't get reset to defaults when renaming columns.

However, renaming columns is only for convenience anyway - the R column names are never user-facing (AFAIK!)

[bobular]

Maybe even do the sync anyway!