process_seed_table: Helper function to process a seed table
In ipfr: List Balancing for Reweighting and Population Synthesis

Helper for ipu(). Strips columns from seed table except for the primary id and marginal column (as reflected in the targets tables). Also identifies factor columns with one level and processes them before mlr::createDummyFeatures() is called.

1	process_seed_table(df, primary_id, marginal_columns)

`df`	the `data.frame` as processed by `ipu()` before this function is called.
`primary_id`	the name of the primary ID column.
`marginal_columns`	The vector of column names in the seed table that have matching targets.