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.

[LourensVeen]

The new build system has per-package development installs available.