FEM Mass matrix element assembly kernel

[artv3]

Kernel for element local mass matrix assembly

[artv3]

@rhornung67 @MrBurmark could I get a review on this PR?

[artv3]

@rhornung67 I added some notes on the z-loops with an iteration space of [0,1) that the kernels have. Let me know if it makes sense; I can expand more.

[MrBurmark]

My opinion on the "extra" length 1 loops is that if they are not necessary in some of the Base/Lambda implementations its fine to remove them. It seems like in this case that is how you would naturally code up the native variants of the implementation. On the other hand its important for the RAJA implementation to be single source and the same for all RAJA variants because that is how we write RAJA code.
To expound a little further afield, and this is not the case here, but sometimes the sequential optimization is large enough that the sequential and parallel implementations are substantially different. In that case its probably best to have two kernels instead of having significant differences between the Base and RAJA variants. An example of this is the basic INDEXLIST and INDEXLIST_3LOOP kernels where there are two ways to create the same list of indices. Breaking this into two kernels allows us to do variant to variant comparisons that are apples to apples and kernel to kernel comparisons that show the performance implications of each algorithm/implementation.

[artv3]

TODO: Remove extra loop in host base variants with iteration range [0, 1)