Monomer groups with explicit connections support in Indigo model and KET-format

**Background**
Indigo macromolecules support in scope of HELM 2.0

**Solution**
Modify Indigo model to support monomer groups and explicit connections (from monomer to monomer with specified attachment points). Add KET export and import.
Tests should include MOL<>KET conversions.

The issue names the Indigo model, KET import/export, monomer groups, and explicit connections, but no files or tests. Start by locating the model and KET format entry points, then review existing MOL↔KET conversion tests. Done means the model supports the requested connections and MOL↔KET conversions are covered by tests.

Written by the indexing model from the issue text.