SIMD unittests [WIP]

[JacobDomagala]

Related to https://github.com/NexGenAnalytics/simd-math/issues/1

TODO:

• [x] Add gtest
• [x] Create src/test directories and add CMake support
• [ ] Add tests

[fnrizzi]

Unit tests need to work on:

• [ ] cover all public functionality of the SIMD classes and its child classes.
• [ ] X86 CPUs
• [ ] ARM CPUs
• [ ] IBM Power CPUs
• [ ] NVIDIA GPUs