The inner classes shouldn't be implemented in the header

re #12041

This reverts commit f6b01f27.

Refs #10377

Add unit test for this

Refs #10377

This reverts commit a6952dc6.

This reverts commit 0a8b9e30.

Group workspaces don't indicate via the logs that they have finished
processing. Normal execution does. I've fixed that here. I've also added a
message about the fact that group processing completion has been done so
that timings can be seen to relate to the overall processing. This will
help where the native processGroups is used, and algorithm execution logs
are reported in addition.

Tests put into place to verify process groups.

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.