Error norms

[kkhemc]

Here are some additions to Kripke that allows it to compute error norms for the type "i" problems, i.e. no scattering. You would run kripke with something like
kripke.exe --sigs 0,0,0 --zones 200,400,200 --quad 8:4 --niters 1 --compute_errors

If this is useful, we could add diagnostics comparing to the angle integrated total fluxes for the "type ii" problems. The added code has the necessary benchmark data, we would just need to add the specific diagnostic calculations.

[ajkunen]

This looks really good, just a few things would be great:

1. I'm getting a number of compiler warnings, mostly related to the need to convert some initializer lists to use double curly braces (for subobject initialization)
2. It would be nice if the --compute_errors flag was added to the --help output
3. It would be nice if there was some mention of it in the README file, with the note about running with sigs=0
4. This doesn't compile on GPU machines. Two options: either make it work on the GPU (this may be difficult) or make it a optional feature that can be controlled via CMake (like -DENABLE_COMPUTE_ERRORS). That way we can turn it off (and not compile it) for GPU builds

[kkhemc]

I believe I have addressed all the issues in the review.

I made some changes so that the errors work on gpu builds. Basically I added policies for ErrorNorms that are sequential when GPU builds are enabled. I built and tested these on lassen.

Please let me know if there are more corrections and/or additions to make.

[ajkunen]

@kkhemc Thanks! I'll try and take a look at this over the weekend or next week.

[brunner6]

@ajkunen and @rchen20 : This MR is really old. If this is still worth merging, we should. Otherwise we should close this. @kkhemc : I'm sorry this has taken more than a few years to evaluate...