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

**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](https://github.com/E3SM-Project/scream/pull/578#discussion_r489128565).

**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.