Create separate function for writing POSCARs and VASP input sets into different directories

[dandavies99]

This is currently handled in the same function that generates all the unique slabs generate_all_unique_slabs. It could be separated out into its own function that can then be called separately.

[brlec]

Is this still relevant with how get_all_slabs and get_one_hkl_slabs are set up now?

[dandavies99]

This has now been addressed on the develop branch in the generation module.