pdn: throw error on missing grid when adding components instead of silent pass

[gadfort]

Summary

Ensures the power grid definition will generate an error when an undefined grid is referenced. This avoids nasty PDN errors when typos appear in the grid component commands and there is no error.

Type of Change

• Bug fix

Impact

For most users a warning will appear when a unused grid is created, but unless they have setup errors, there should be no change to the final result.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass.
• My code follows the repository's formatting guidelines.
• I have included tests to prevent regressions.
• I have signed my commits (DCO).

[gemini-code-assist]

Code Review

This pull request introduces dummy instance grids to handle cases where no instances are found for a macro grid definition, preventing hard failures and instead issuing a warning. Key changes include the addition of the DummyInstanceGrid class, updates to grid retrieval and lookup methods to support dummy grids, and integration into the Tcl grid definition flow. Two critical issues were identified in the Tcl implementation: an invalid empty command evaluation [] used for list initialization, and a SWIG type mismatch caused by passing a list of domains instead of iterating over them individually.