Std experimental simd

[srinivasyadav18]

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name: Adapt experimental::simd to octotiger
about: New simd backend for vectorization

Proposed Changes

• Adds new cmake flag OCTOTIGER_WITH_STD_EXPERIMENTAL_SIMD which enables to use std::experimental::simd datatypes
• Adds new std_simd.hpp for handling simd data types.

[G-071]

The Kokkos CI Jobs are failing since they're running without std::simd, yet this flag is on by default!

I propose turning it OFF by default (for now) until it becomes less experimental. If you can give me push access to your fork, I can quickly add a new Jenkins Pipeline that additionally builds and tests everything with std::experimental::simd enabled as to prevent this from breaking in the future until we hit the "less experimental" stage (I would simply require your build command and push access).

[G-071]

LGTM -- My local tests with the std-simd backend enabled passed (as do the Jenkins ones as long as keep Kokkos SIMD as the default for now).

I will add the Jenkins pipeline for this in a separate PR!