Made two tests for potential mapping with the 'setups' parameter and pulled out a function from the huge GenerateVaspInput class.

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.

Summary

In an effort to tackle #1340 I tried to refactor POTCAR writing. Did not get very far before it was decided that there are probably more worthy tasks. However, I added a couple of tests which deal with potential mapping via the setups key in the Vasp calculator.

I also added some future warnings for the usage of the PW91 pseudopotentials. They are very old, no longer supported and should not be used! This is also an issue in the documentation where the setup described to make potentials available for ASE speaks about 3 pseudopotential libraries (potpaw for LDA, potpaw_GGA for PW91, and potpaw_PBE for PBE). This has been outdated since 2012 with the release of potpaw.52. The current potential set (potpaw.64) includes only potpaw_LDA and potpaw_PBE. I think the documentation should be updated.