Vector backend using UME::SIMD

This merge request is a copy of: Merge Request #81

Due to rebase problems, keeping original branch would cause history to be obscured.

Umesimd integration

This change introduces new vectorization backend using UME::SIMD library. There are still few things in the code that I want to change, so this is not an immediate merge request.

Since I am new to the project, I would like to get some feedback on:

coding rules
predicate correctness in volume implementations
whether build system can be broken for any build configuration

UME::SIMD can be found at: https://bitbucket.org/edanor/umesimd/overview Please use the most recent stable release.