feature/avx512 float

Issue/feature addressed

We have only implemented scalar and avx2 backends for floats

Proposed solution

Implements avx512 backend for floats

Implementation

...

Tests

Minor modifications to the existing unit tests was needed

Notes

This MR depends on !1255 (merged) SVE and SSE2 still need to be implemented

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).

Warning

On the 19.07 the code formatting (code style) was standardised using clang-format, over the whole Nektar++ code. This means changes in your branch will conflict with formatting changes on the master branch. To resolve these conflicts , see #295 (closed)