Add support for named point definitions

Summary

Allows the definition of named point sets. This is the prerequisite for a future MR that will allow ComIn plugins to access grids and points defined by ICON.

Related Issues

• Related #262 (closed)

Documentation

• updated reference.dox

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 successful
• Update changelog, if changes are worth mentioning (Develop section in doc/src/changelog.dox, add section if not yet included)