REUSE compliance

[ClaudiaComito]

Context
We want to improve the maturity of our license compliance (Reusability dimension of the Helmholtz Quality Indicators for Research Software). This means ensuring that Heat has a clearly defined, machine-readable, and automatically enforced licensing structure. Eventually, the upgrade will also make it possible for single modules to be licensed individually, opening up Heat usage to domains or industries that require more restrictive licensing.

Feature functionality

See https://reuse.software

• Implement file-level licensing
• integrate the reuse linting tool into our CI

see https://reuse.software/dev/

Additional context
Task 2.2 of the HeatHub project.