Make environment_setup in configuration optional

[luator]

Currently, all options inside the environment_setup section are optional, but the section itself is required. So even if not setting anything there, one needs to add an empty environment_setup in the configuration file.
This is a bit annoying, so I suggest to make the whole section optional.

[luator]

Same for fixed_params.

[luator]

Once done, update the tutorials (they currently contain these sections with comments that they have to be there, even if empty).