refs #7305. Add docs for usage, description, summary and props.

Also move memory check into MDHisto routine, where it belongs. Memory check is based on a predictive MDHistoWorkspace size, so gives false results for MDEventWorkspaces. Also switched AdaptiveBinned property default to on, since this is the preferred loading route.
Merge remote-tracking branch 'origin/feature/7305_loadvtk_docs'