Add option to NumNonlinearIterations to accumulate over step ref#4052

Added new option “accumulate_over_step” to the NumNonlinearIterations
Postprocessor.  When set to true, this returns the sum of the nonlinear
iterations for all Picard iterations in a step.
Merge pull request #4053 from bwspenc/accumulate_nl_its

Add option to NumNonlinearIterations to accumulate over step