check_tables: Check seed and target tables for completeness
In pbsag/ipfr: Develop Expansion Weights for a Survey

Given seed and targets, checks to make sure that at least one observation of each marginal category exists in the seed table. Otherwise, ipf/ipu would produce wrong answers without throwing errors.

1 2	check_tables(primary_seed, primary_targets, secondary_seed = NULL, secondary_targets = NULL)

`primary_seed`	In population synthesis or household survey expansion, this would be the household seed table (each record would represent a household). It could also be a trip table, where each row represents an origin-destination pair. Must contain a `pid` ("primary ID") field that is unique for each row. Must also contain a geography field that starts with "geo_".
`primary_targets`	A `named list` of data frames. Each name in the list defines a marginal dimension and must match a column from the `primary_seed` table. The data frame associated with each named list element must contain a geography field (starts with "geo_"). Each row in the target table defines a new geography (these could be TAZs, tracts, clusters, etc.). The other column names define the marginal categories that targets are provided for. The vignette provides more detail.
`secondary_seed`	Most commonly, if the primary_seed describes households, the secondary seed table would describe a unique person with each row. Must also contain the `pid` column that links each person to their respective household in `primary_seed`. Must not contain any geography fields (starting with "geo_").
`secondary_targets`	Same format as `primary_targets`, but they constrain the `secondary_seed` table.