fix serialization in StableConsistencyModel

[arrjon]

• Enhanced the FourierEmbedding class with a get_config and from_config methods for full serialization of layer parameters. Before, an error was raised when loading a saved StableConsistencyModel.
• Added a concatenate_subnet_input flag to the StableConsistencyModel constructor, allowing users to choose whether subnet inputs are concatenated into a single vector or passed as separate arguments like for the other free form architectures.

[codecov]

Codecov Report

❌ Patch coverage is 25.71429% with 26 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...able_consistency_model/stable_consistency_model.py	8.69%	21 Missing ⚠️
bayesflow/networks/embeddings/fourier_embedding.py	58.33%	5 Missing ⚠️

Files with missing lines	Coverage Δ
bayesflow/networks/embeddings/fourier_embedding.py	84.37% <58.33%> (-15.63%)	⬇️
...able_consistency_model/stable_consistency_model.py	20.32% <8.69%> (-1.11%)	⬇️