Make Imas dependency optional for the server

[khalsz]

Imas_python should be an optional dependency in the pyproject.toml file, hence should not be in the base (main) dependency.

[olivhoenen]

Hi @khalsz, this was discussed in the developer's meeting. We now better understand the point, which is more related to the server side. For the client, while some use-cases will not rely on IMAS, there is no way to say in advance whether a client user will ingest a simulation that contains IMAS output or not. And as IMAS-Python now brings automatically IMAS-Core and is relatively lightweight compare to other deps such as SciPy/NumPy, we have decided to keep this as a mandatory dep for the client.

So maybe here the issue is more to separate the core deps from both client and server deps, if that makes sense?