Refactor workspace group code

[mantid-builder]

Original Reporter: Russell Taylor

Very generic title I know. The specific reason for creation is to look at whether the code commented out of BinaryOperation in http://trac.mantidproject.org/mantid/ticket/4630 can be restored.

More generally, the workspace group code has been observed to be very fragile and is exceptionally verbose, with what looks like a lot of code repetition.
An example of a problem: If an algorithm on a group fails, you end up with an empty output workspace group.

This is related to http://trac.mantidproject.org/mantid/ticket/996 & http://trac.mantidproject.org/mantid/ticket/4625.

[mantid-builder]

This issue was originally trac ticket 4635