Group datasets by experiment PLOTTING label

Created by: wilsonmr

closes #413 (closed), closes #418 (closed)

closes #174 (closed) (the dataspecs plots already exists)

At the moment this is EXTREMELY basic, but it explains better my approximate approach to addressing the problem that the plots by experiment in vp reports are essentially redundant.

Instead of rewriting the various actions (another possibility which shouldn't be discounted/forgotten) I aimed to keep the idea of experiment as some arbitrary combination of datasets. There is a production rule which for a given fit, looks at the plotting info and then groups those datasets into experiments. Then one can do something like

{@with experiments_from_plotting@}
{@plot_experiments_chi2@}
{@endwith@}

and the plot will have bars according to the plotting experiments. I then added a provider fixup_fitthcovmat which handles the loading of the fitthcovmat and fixes the experiment headers to match those of the experiments. Then anything which used fitthcovmat now uses fixup_fitthcovmat and in DataResult I now take the cross section of fixup_fitthcovmat for either experiment or dataset.

The final step would be introducing some minimal #356 (closed) so that one can calculate total chi2s required for some tables and the summarise_fits action. All of this is currently a bit gross at the moment, can probably be done in a cleaner way, or in fact I could stop with this approach and try something completely different, it has however produced this: (results include th covmat and may or may not be correct - they look rather sensible compared to not using th covmat but that's not very rigorous)