repop mode can also start from CSV files instead of one HDF5 file

[bstabler]

We have a use case where we want to repop a third party synthetic population. In order to do so, we would need to create the seed synthetic population hdf5 datastore, However, it would be easier to repop if we could create the seed datastore from CSV files. It may not be worth the effort to add this functionality, but let's think about it.