development install method has changed

[rieder]

(discussed with @ipelupessy before on Slack, archiving / continuing here) See https://setuptools.pypa.io/en/latest/userguide/development_mode.html

Previously, pip install -e . would only install AMUSE for pip, it would not build anything. That default behaviour has changed now. It can (for now) be restored to the old behaviour, by setting SETUPTOOLS_ENABLE_FEATURES="legacy-editable". We would need to solve this in a more permanent way at some point though.