This is more in line with how the new functions work and how the C++
side works too. Refs #970

There is now a single directory called plugins that contains sub-
directories for algorithms & functions. These get installed into the
package under ROOT/plugins/python. This requires new keys to find the
directories and deprecates pythonalgorithms.directories key.

Users will receive warnings about the old key being deprecated and are
instructed about what to do.

Refs #970

It took 19 months to get to. And <30 minutes to fix and test.

The bug was recapturing the workspace as a result of SaveGSS which
does not return anything. The following call to DeleteWorkspace
failed because it was given 'None'.