Refs #6622 Add option to output workspaces created by Fit.

Couple of things to note:
 - The CreateOutput option defaults to false so the algorithm won't
suddenly start churning out lots of workspaces.
 - As the algorithm outputs all the parameters to a single table,
regardless of the number of input workspaces, I've chosen to output all the
output workspaces to a single group per type.
 - Output workspaces are numbered according to the spectrum in the input workspace
used.
Refs #6622 Add unit tests for new option.
Merge branch 'feature/6622_plotpeakbylogvalue_fit_output' into develop