docstring linter

Add docstring linting to CI and splits the linting into three sections (PEP8 style, syntax, and docstring). The docstring linting and the style linting are allowed to fail.

See https://gitlab.astro-wise.org/euclid/datamodelvalidator/pipelines/1132 for the output of the pipeline.

Also fix two minors issue the syntax linter brought to light.