Implement specialized templates for pow<2>, pow<3>, pow<4>.

[jeff-cohere]

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Is your feature request related to a problem? Please describe. In order to make bit-for-bit testing easier, it would be nice to have specialized implementations of the pow function for low integer exponents. In particular, see this conversation.

Describe the solution you'd like C++ template specializations for pow<2>, pow<3>, and pow<4>. Perhaps we should include some Fortran support for these and other functions in EKAT as well.

[bartgol]

We can probably do a template utility for the generic pow<N> (log2(N) recursions). It should be fairly straightforward.

[jeff-cohere]

Do you have any HOMME code or other prior art we can use? Or do you have a new implementation in mind?

[bartgol]

I have an impl for a runtime version; should be immediate to convert to templated (or even add both).

Btw, the bfb_pow_impl function in that file is, imho, a better solution for bfb pow than bridging F90 to Cuda. One might argue that it is expensive, but it might be a wash with the Cuda kernel launch (I never checked though).