[VOLK] Release v2.1.0

[Johannes Demel]

Hi everyone,

we just released VOLK v2.1.0!

we would like to announce that Michael Dickens and Johannes Demel are
the new VOLK maintainers. We want to review and merge PRs in a timely
manner as well as commenting on issues in order to resolve them.

We want to thank all contributors. This release wouldn't have been
possible without them.

We're curious about VOLK users. Especially we'd like to learn about VOLK
users who use VOLK outside GNU Radio.

If you have ideas for VOLK enhancements, let us know. Start with an
issue to discuss your idea. We'll be happy to see new features get
merged into VOLK.

## Highlights

VOLK v2.1.0 is a collection of really cool changes. We'd like to
highlight some of them.

- The AVX FMA rotator bug is fixed
- VOLK offers `volk::vector<>` for C++ to follow RAII
- Move towards modern dependencies
    - CMake 3.8
    - Prefer Python3
        - We will drop Python2 support in a future release!
    - Use C++17 `std::filesystem`
        - This enables VOLK to be built without Boost if available!
- more stable CI
- lots of bugfixes
- more optimized kernels, especially more NEON versions

## Contributors

*  Albin Stigo <address@hidden>
*  Amr Bekhit <address@hidden>
*  Andrej Rode <address@hidden>
*  Carles Fernandez <address@hidden>
*  Christoph Mayer <address@hidden>
*  Clayton Smith <address@hidden>
*  Damian Miralles <address@hidden>
*  Johannes Demel <address@hidden> <address@hidden>
*  Marcus Müller <address@hidden>
*  Michael Dickens <address@hidden>
*  Philip Balister <address@hidden>
*  Takehiro Sekine <address@hidden>
*  Vasil Velichkov <address@hidden>
*  luz.paz <address@hidden>


## Changes

* Usage
    - Update README to reflect how to build on Raspberry Pi and the
importance of running volk_profile

* Toolchain
    -  Add toolchain file for Raspberry Pi 3
    -  Update Raspberry 4 toolchain file

* Kernels
    - Add neonv7 to volk_16ic_magnitude_16i
    - Add neonv7 to volk_32fc_index_max_32u
    - Add neonv7 to volk_32fc_s32f_power_spectrum_32f
    - Add NEONv8 to volk_32f_64f_add_64f
    - Add Neonv8 to volk_32fc_deinterleave_64f_x2
    - Add volk_32fc_x2_s32fc_multiply_conjugate_add_32fc
    - Add NEONv8 to volk_32fc_convert_16ic

* CI
    - Fix AVX FMA rotator
    - appveyor: Enable testing on windows
    - Fixes for flaky kernels for more reliable CI
        - volk_32f_log2_32f
        - volk_32f_x3_sum_of_poly_32f
        - volk_32f_index_max_{16,32}u
        - volk_32f_8u_polarbutterflypuppet_32f
        - volk_8u_conv_k7_r2puppet_8u
        - volk_32fc_convert_16ic
        - volk_32fc_s32f_magnitude_16i
        - volk_32f_s32f_convert_{8,16,32}i
        - volk_16ic_magnitude_16i
        - volk_32f_64f_add_64f
    - Use Intel SDE to test all kernels
    - TravisCI
        - Add native tests on arm64
        - Add native tests on s390x and ppc64le (allow failure)

* Build
    - Build Volk without Boost if C++17 std::filesystem or
std::experimental::filesystem is available
    - Update to more modern CMake
    - Prevent CMake to choose previously installed VOLK headers
    - CMake
        - bump minimum version to 3.8
        - Use sha256 instead of md5 for unique target name hash
    - Python: Prefer Python3 over Python2 if available

* C++
    - VOLK C++ allocator and C++11 std::vector type alias added