Generate getter design for WorkspaceGroups

[OwenArnold]

This issue was originally TRAC 10501

This ticket is blocks : TRAC10503

For workspace groups:

1. Where we have multiperiod data (identified by existing isMultiperiod() method), we ought to have forwarding functionality to get the run_number, nhistograms, blocksize from the group.

Easy enough to do in python. Should this also be a c++ feature?

2. A mechanism for a generic group workspace to extract all the run numbers from a group as a list in python.

This needs to go to the SSC, possibly with a design.

[raquelalvarezbanos]

From Owen: I think the proper answer to this is related to what we discussed with @martyngigg at the Developer workshop 2016 regarding workspace types. A design document for this is pending.