RenameWorkspaces should only overwrite if forced

[NickDraper]

Expected behaviour

RenameWorkspace should fail if an existing workspace of the output name is defined. This check needs to be for anything in the ADS and also members of groups in the ADS.

We should add a new overwite Boolean property (default false) that would delete the outputworkspace if it exists before renaming.

Actual behaviour

Currently renameWorkspace can overwite workspaces if the outputworkspace name is already in use.
This can make a particular mess if you name two members within a workspace group the same (see #8652).

Steps to reproduce the behavior

CreateSampleWorkspace(OutputWorkspace='bob', Function='User Defined', UserDefinedFunction='name=LinearBackground,A0=3.22542,A1=-0.0633193')
CreateSampleWorkspace(OutputWorkspace='bob1', Function='User Defined', UserDefinedFunction='name=LinearBackground,A0=3.22542,A1=-0.0633193')
CreateSampleWorkspace(OutputWorkspace='bob2', Function='User Defined', UserDefinedFunction='name=LinearBackground,A0=3.22542,A1=-0.0633193')
CreateSampleWorkspace(OutputWorkspace='bob3', Function='User Defined', UserDefinedFunction='name=LinearBackground,A0=3.22542,A1=-0.0633193')
GroupWorkspaces(InputWorkspaces='bob,bob1,bob2,bob3', OutputWorkspace='NewGroup')
RenameWorkspace(InputWorkspace='bob1', OutputWorkspace='bob')

This will give you a group with two members with the same name, there is lots of trouble from this point on.

Platforms affected

All

Once this is solved we should check with Rob Dalgleish to make sure this does not mess up his scripts.