Add check to stop upload of redundant plots

Created by: wilsonmr

After reading the email from @Zaharid, specifically:

• Please do not use the same output folder for all of the validphys reports. Note that a warning is printed when the output exists, and there is the -o option to select a different one. Not only dies this interfere with the thumbnail generation algorithm, but storing a copy of all the vp plots you have ever produced for each report involves a not so insignificant amount of wasted resources (considering these things are backed up in several places).

I was wondering if it wouldn't be too difficult to add a check which looked in the /figures/ folder and checked that all of the plots are declared in the report output using something like: https://stackoverflow.com/a/4295213

Then maybe stop upload if it wasn't satisfied.. Unless I am missing a potential problem this could introduce or perhaps this would make uploads significantly slower and is overkill?