Allow controlling number of particles with a postprocessor

[meltawila]

closes #1237

[moosebuild]

Job Documentation, step Sync to remote on ddc8214 wanted to post the following:

View the site here

This comment will be updated on new commits.

[moosebuild]

Job Coverage, step Generate coverage on ddc8214 wanted to post the following:

Coverage

	06d42a	#1239 ddc821
	Total	Total	+/-	New
Rate	93.08%	93.09%	+0.00%	100.00%
Hits	9378	9383	+5	9
Misses	697	697	-	0

Diff coverage report

Full coverage report

This comment will be updated on new commits.

[aprilnovak]

Nice addition! This is clean and relatively unintrusive to the code - just some minor style & testing comments from me.

[meltawila]

The VTB test failure is unrelated

[nuclearkevin]

PR to fix the VTB failure is up. VTB tests should start passing one this is merged.

[moosebuild]

Job Test VTB on ab4fc4f : invalidated by @meltawila

[aprilnovak]

thanks @meltawila this looks good after these last comments LGTM!

[moosebuild]

Job Precheck, step Clang format on c5e7bc4 wanted to post the following:

Your code requires style changes.

A patch was auto generated and copied here
You can directly apply the patch by running, in the top level of your repository:

curl -s https://mooseframework.inl.gov/cardinal/docs/PRs/1239/clang_format/style.patch | git apply -v

Alternatively, with your repository up to date and in the top level of your repository:

git clang-format 06d42a417d2f69c651cd798a2f6df39c21fad3e2