Stackinator

Lots of new features, and bug fixes.

Features:

* Remove cluster configuration from the stackinator repo so that clusters can be managed separately (99)
* Support for environment views (101)
* Simplified build cache configuration (102)
* Support for CPE 23.03 and cray-mpich 8.1.25 on Alps (105)
* Created complete online documentation (107)
* https://eth-cscs.github.io/stackinator/
* Add a very useful debug script for live-debugging partially built environments (114)
* Integration of changes from the `develop` branch of Spack (122)
* Support for post-install and pre-install hooks (131) and (135)
* Support pushing partially built.

Features:
* Add headers and libraries to Cray-MPICH (87)
* add cray-mpich-8.1.24 from cpe-23.02 (83)
* Allow user-provided packages (75)
* add --system flag to override system in config.yaml (67)

Upgrade system configuration after the Slingshot upgrade:
* update external xpmem version (88)
* Move cluster config, update libfabric paths, add rdma-core back (85)

Tweaks:
* Improve schema for environments.yaml (80)
* remove default hohgant cache (81)
* Print system only if provided in the CLI (74)
* Format with black and isort (70)
* Remove obsolete cray-mpich-binary package.py (64)

Fixes:
* Fix flake8 and mypy complaints (72)
* Test that build path is not root or in tmp (73)
* Fix python decoding errors when reading files (79)
* don't copy .git path to recipe meta to avoid configuration errors (68)

The first official release of the stackinator.
Available via `pip install stackinator`.