Update the nebular models to extend the list of metallicities and address a few issues in the list of lines.

Improve the description of the data file format by specifying how to indicate upper limits and missing values.

Import Iterable from collections.abc rather than from collections in order to prevent ImportError/AttributeError exceptions with Python 3.10+.

Revert "Partially revert "Import changes from commit 3d1c0d76 so that pcigale is installable (credits Yannick Roehlly).""

This reverts commit 6b82dbe3.

This reverts commit 3fb010a6.

Partially revert "Import changes from commit 3d1c0d76 so that pcigale is installable (credits Yannick Roehlly)."

This reverts commit 00d3c2ff.