* xtensor-python: add version 0.23.1
* xtensor-python: update xtensor/xtl dependency version constraints
for 0.23.1
* xtensor-python: remove xtensor/xtl dependency version constraints
for the develop version to favor the latest (develop) version of
these dependencies
* xsimd: add version 7.2.3
* xtensor: add tbb variant to enable TBB parallelization
* xtensor: add version 0.20.7
* xtensor: update CMake args enable xsimd/tbb support based on
variant settings
* xtl: add version 0.6.4
- remove the old LGPL license headers from all files in Spack
- add SPDX headers to all files
- core and most packages are (Apache-2.0 OR MIT)
- a very small number of remaining packages are LGPL-2.1-only
