[VOLK] Maintenance release 3.1.1

Hi everyone!

This is the VOLK v3.1.1 release! We want to thank all contributors. This
release wouldn't have been possible without them.

This is a maintenance release to fix subtle bugs in many areas and to
improve our tests where possible. All in all, our CI is more stable now
and catches more errors.

### Contributors

- Clayton Smith <argilo@gmail.com>
- Johannes Demel <demel@uni-bremen.de>, <jdemel@gnuradio.org>
- Kenji Rikitake <kenji.rikitake@acm.org>
- Philip Balister <philip@opensdr.com>

### Changes

- CI fixes
- Allow for rounding error in float-to-int conversions
- Allow for rounding error in `volk_32fc_s32f_magnitude_16i`
- Allow for rounding error in float-to-int interleave
- Add missing `volk_16_byteswap_u_orc` to puppet
- Fix 64-bit integer testing
- Build and test neonv7 protokernels on armv7

- kernels
- Remove broken sse32 kernels
- Fix flaky `fm_detect` test
- Fix flaky `mod_range` test
- Remove unnecessary volatiles from `volk_32fc_s32f_magnitude_16i`
- Remove SSE protokernels written in assembly
- Remove inline assembler from `volk_32fc_convert_16ic_neon`
- Use bit shifts in generic and `byte_shuffle` reverse
- Remove disabled SSE4.1 dot product
- Fix `conv_k7_r2` kernel and puppet
- Remove unused argument from renormalize
- Align types in ORC function signatures
- Uncomment AVX2 implementation
- Renormalize in every iteration on AVX2
- Remove extraneous permutations
- Compute the minimum over both register lanes
- `volk_32fc_s32f_atan2_32f`: Add NaN tests for avx2 and avx2fma code

- fixes
- Express version information in decimal
- Remove `__VOLK_VOLATILE`
- Remove references to simdmath library
- cmake: Switch to GNUInstallDirs
- fprintf: Remove fprintf statements from `volk_malloc`
- release: Prepare release with updated files
- Get the sse2neon.h file to a git submodule to avoid random copies.