Incremental builds

[gipert]

Editable installs (which imply incremental builds) are not fully supported by scikit-build-core yet: scikit-build/scikit-build-core#114

In the meanwhile, caching across repeated builds can be achieved by commenting out requirements in pyproject.toml.build-system.requires and installing them manually.

If calls to pip install fail (see #49), a solution is to remove the build/ directory or commenting out build-dir in pyproject.toml (will then use a different, temporary build directory every time).