HistogramVectorPostprocessor only works with VPPs whose vectors are defined at construction

[bwspenc]

Bug Description

HistogramVectorPostprocessor computes a histogram from the values in the vectors computed by another VectorPostprocessor. The HistogramVectorPostprocessor declares its vectors based on the names of the vectors in the other VPP at construction time. If those are not yet created at that time, it errors out.

Steps to Reproduce

Use HistogramVectorPostprocessor with a VPP that delays its creation of vectors. I ran into this problem in Grizzly with a VPP that has to delay when it declares its vectors because the number of vectors is only known at the initialize() phase. I don't know whether there are VPPs like that in MOOSE.

Impact

This is preventing us from using this VPP.

[bwspenc]

@hoffwm @aeslaughter

[bwspenc]

The fix for this is pretty simple -- we just move some code that was run at construction to initialize(). We tested that and it works fine. I'm just not sure whether that's an acceptable solution. @aeslaughter, maybe you have thoughts on that.

[bwspenc]

Turns out that we can actually solve this problem by having our other VPP figure out its vectors at construction time. However, I'm keeping this ticket open because we should improve the error behavior of this. Currently, this is the error message you get if your other VPP doesn't have any vectors:

application called MPI_Abort(MPI_COMM_WORLD, 1) - process 0
[unset]: write_line error; fd=-1 buf=:cmd=abort exitcode=1
:
system msg for write_line failure : Bad file descriptor

We need to change this to give an error message that actually tells you what is going on here.