Added a post-processor to compute relative difference between 2 post-processors

[joshuahansel]

Added a post-processor to compute relative difference between 2 post-processors.

Closes #10148

[moosebuild]

Job Documentation on 8be1b6a wanted to post the following:

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[dschwen]

Linear combination, relative difference... looks like it is starting to make sense to add a ParsedPostprocessor

[permcody]

Let's just jump right to ParsedKernel. In fact, we'll just let you write your weak form in a simple python script...

[YaqiWang]

I believe I once required this ParsedPostprocessor but ended up with a ScalePostprocessor.

[permcody]

FYI - We coded this common operation up a long time ago:
https://github.com/idaholab/moose/blob/devel/framework/include/utils/MooseUtils.h#L213

[permcody]

Oh sorry - that's not the correct method. You are comparing against zero here. There are several methods in that file you might want to check out.

[joshuahansel]

Ok thanks, I will use absoluteFuzzyEqual then.

[dschwen]

Let's just jump right to ParsedKernel. In fact, we'll just let you write your weak form in a simple python script...

My new function parser will actually make a parsed kernel totally possible! With actual jacobian derivation.

[joshuahansel]

Let me know when Santa brings us a ParsedPostprocessor. In the end what I'd really like is a ParsedTester that works on post-processor values directly instead of going through an output file...