[IMPROVEMENT] Add metadata to PGM-DS serialization

[vincentkoppen]

Problem faced

Currently there is no metadata in the json serialization of PGM-DS. Might be good to add this.
(Separated from #288)

Improvement proposal

We could introduce some basic metadata, similar to the one PGM has.
So for example

{ 
   "version": "1.0", 
   "type": "ds", 
   "data": [...]
}

Community Meeting

• I wish to discuss this improvement in the bi-monthly PGM community meeting. Please join the PGM community meeting.

[Saim-naushad]

Hi Vincent, I’d be interested in working on this. I previously contributed #303 in the same serialization module, so this would be a useful continuation of that work.

I reviewed the discussion in #288 and understand that the goal here isn’t to make PGM-DS serialization interchangeable with regular PGM input data. It’s to identify and version the PGM-DS format while preserving its additional grid data.

My proposed scope would be to add the agreed top-level metadata through serialize_to_dict(), which would cover file and in-memory serialization, while keeping deserialization compatible with existing files containing only data. I would also add regression tests for both the new and legacy formats.

Before I start, should the new format contain only version: "1.0" and type: "ds" as shown in this issue, or should it also include is_batch and attributes? Should deserialization validate the version and type, or remain permissive for now?

If that scope sounds right, I’d be happy to work on it.