New feature observation

[adrianarce-elemwave]

Observation Module Capabilities

This PR introduces and validates the current observation subsystem capabilities for storing, updating, and flushing FDTD probe data.

The observation module currently supports:

• Structured storage for time-domain and frequency-domain probe data.
• Allocation and cleanup helpers for serialized observation buffers.
• Time-domain scalar and vector field storage.
• Frequency-domain complex field storage.
• Serialized current metadata storage, including cell indices, current type, and material/tag id.
• Observation preprocessing through preprocess_observation.
• Reset of observation state flags: done, begun, and flushed.
• Clamping and correction of observation time windows.
• Normalisation of invalid or excessive frequency steps.
• SaveAll handling for non-volumic probes.
• Special handling for volumic probes and mapvtk observations.
• Observation initialisation through InitObservation.
• Allocation of per-request and per-probe output structures.
• MPI-aware clipping or disabling of probes outside the local subdomain.
• Output path and file-unit setup.
• Duplicate output filename handling.
• Initialisation of volumic binary outputs.
• Initialisation of point, block-current, wire, charge, line-integral, far-field, and map-VTK style probe metadata.
• Runtime updates through UpdateObservation.
• Point electric and magnetic field probes.
• Block electric and magnetic current probes.
• Wire current and charge probes.
• Line-integral probes.
• Volumic scalar and component field probes.
• Volumic current and map-VTK serialisation.
• Frequency-domain accumulation using DTFT-style complex updates.
• Optional MTLN probe initialisation, update, and close paths when compiled with MTLN support.
• Output flushing through FlushObservationFiles.
• ASCII time-domain probe output.
• Binary volumic output.
• Frequency-domain binary output.
• Incident-field output columns when plane waves are present.
• MPI synchronisation for block-current outputs.
• Far-field flushing.
• Resource cleanup through DestroyObservation.
• Access to observation output state through GetOutput.
• Field-to-storage mapping through fieldo.
• DTFT computation through dtft.

Test Coverage Added

The current test coverage includes:

• Allocation/deallocation tests for Serialized_t time-domain buffers.
• Allocation/deallocation tests for Serialized_t frequency-domain buffers.
• Allocation/deallocation tests for serialized current metadata.
• Preprocessing tests for initial-time correction.
• Preprocessing tests for mapvtk time-window handling.
• Preprocessing tests for timestep larger than the requested observation window.
• Preprocessing tests for zero or oversized frequency steps.
• Preprocessing tests for volumic and non-volumic SaveAll behaviour.
• Preprocessing tests for SaveAll time-window expansion.
• Preprocessing tests for final-time correction when it is earlier than initial time.
• Preprocessing tests for protection against excessively large time ranges.
• Initialisation smoke coverage for a volumic time-movie observation.
• Update smoke coverage for a volumic time-movie observation.
• Output metadata/path validation for the volumic time-movie initialisation path.