refs #10131. Worker class put in place.
refs #10131. Implement checkGroups

I've actually called it what it is instead. findMultiPeriodGroups. I also really dislike the confusing side effects caused by the existing Algorithm::checkGroups, so I've decided not to go with that design, and instead have findMultiPeriodGroups as a const method.
refs #10131. Implemented processGroups.

Tests put into place to verify process groups.
refs #10131. MultiPeriodWorkspaceAlgorithm refactored.

Now uses worker instance.
Merge branch 'feature/10131_process_groups' into develop