Avoid overflow for volume fraction calculation

[BradWhitlock]

This is a follow-on for #1875 (merged into develop now).

This PR resolves #1871 by:

• Changing MFEM shaping helper shaping::computeVolumeFractionsForMaterial so it works on groups of elements rather than all elements at once when it looks like the number of values will overflow int32
• Passes execution policy to shaping::computeVolumeFractionsForMaterial so we can parallelize for OpenMP some of the more expensive algorithm phases

Shaping driver varying mesh size for spheres.yaml input with order 4 output

[kennyweiss]

Thanks @BradWhitlock

It's a bit difficult to tell from the github review page, but it looks like the shaping_helpers_mfem no longer runs on device, where previously several parts (but not all of it) ran on device, when using a hip or cuda policy.
Can you please run this in a GPU config/policy and confirm that we're not losing functionality? The speedups were significant in some of my runs, and getting the memory transfers correct/working was challenging.

Update: I don't think any of our hip/cuda configs are currently using mfem::Device functionality. Outside of this PR, we should enable/exercise mfem on-device for our shaping problems in our hip/cuda configs.
I created this issue to track this: #1966

[kennyweiss]

Minor: Please update the doxygen for this function

[kennyweiss]

Please add a comment to this line.
Presumably, it is sized to avoid overflow w/ int32.

Should we upstream a concern/fix to mfem about overflow w.r.t. int32 vs int64 indices?

[kennyweiss]

Perhaps it would be sufficient (or at least an improvement) for mfem to check for and give a warning about overflow?

[kennyweiss]

Should we warn the user about this?
Our implementation (in Axom/MFEM) is only on the CPU, so there's nothing they can do about it.

If we do want to post a warning, perhaps it would be sufficient to only warn once in a given run?
E.g. using SLIC_WARNING_ROOT_ONCE

axom/src/axom/slic/interface/slic_macros.hpp

Lines 237 to 255 in 12695a3

	/*!
	* \def SLIC_WARNING_ROOT_ONCE( msg )
	* \brief Macro that logs given warning message only on root, and only once per call site.
	*
	* \param [in] msg user-supplied message.
	*
	* \note The SLIC_WARNING_ROOT_ONCE macro is always active.
	* \note By default, all ranks are considered to be root.
	* Must call `axom::slic::initialize(is_root={true|false})`
	* or set via `axom::slic::setIsRoot({true|false})` to filter based on root.
	*
	* Usage:
	* \code
	* SLIC_WARNING_ROOT_ONCE( "A warning has occurred!" );
	* \endcode
	*
	*/
	#define SLIC_WARNING_ROOT_ONCE(msg) \
	SLIC_DETAIL_LOG_IF_ONCE(SLIC_WARNING_IF, axom::slic::isRoot(), msg)

[kennyweiss]

I'm reasonably sure that the FCT ran on device.

Update: Nevermind -- this never ran on device

axom/src/axom/quest/detail/shaping/shaping_helpers_mfem.cpp

Lines 694 to 699 in 12695a3

	AXOM_ANNOTATE_BEGIN("fct project");
	axom::for_all<axom::SEQ_EXEC>(0, NE, [=](int i) {
	FCT_correct(&m_d(0, 0, i), dofs, &b_d(0, i), minY, maxY, &vf_d(0, i), &fct_mat_d(0, 0, i));
	});
	AXOM_ANNOTATE_END("fct project");
	}