Create statistics VPP for stochastic simulations

[cbolisetti]

Currently the user has to post process the results from a stochastic simulation. This VPP will calculate the mean, median and envelopes of the VPP values provided by a VPP name.

[Vectorpostprocessors]
  [./accel_hist_6x_statistics]
    type = MeanMedianEnvelopeCalculator
    vectorpostprocessor = 'accel_hist_6x'
    execute_on = timestep_end
  [../]
[]