fix yac.core with --disable-utils

Summary

yac.core bindings dont compile with --disable-utils. See https://buildbot.dkrz.de/#/builders/21/builds/8233/steps/2/logs/ICON

This MR

• introduces a test that builds YAC with --disable-utils
• fixes the issue by providing fallback implementations for read_icon and read_scrip in core.pyx. This implementations simply die and print an error message.

Checklist

• All code changes are covered by unit tests
• Code coverage did not deteriorate
• Code follows Style Guide
• Documentation updated, if necessary
• WIP: Manual running of configure/build jobs was successfull
• Update version info, if changes are worth mentioning (Develop section in doc/src/version_info.dox, add section if not yet included)