[Feature] Ability to read multiple files and obtain true uncertainties

[drewejohnson]

Create a new set of classes that are responsible for processing multiple output files of the same type and storing the true uncertainties.

Requirements

• Each class should be given a list of files at construction
• Support globs, '*'
• Store metadata when applicable - things that are invariant through repeated runs
• Store files used
• Store average value and uncertainty for any quantity of interest, e.g. material number densities, cross sections for branching calculations, detector tallies, etc.

Optional

• Store seeds used - optional argument at construction seed=True

Things to consider

• Would it be advantageous to also retain all the data used, in a manner than allows people to identify runs with particularly large uncertainties?
• Some plot commands will have to be modified, e.g. depletion reader plot -> error bars