Expand to include forward modeling methods

[corentinravoux]

Large rework of flip:

• covariance will be a subpackage inluding the full field-level likelihood-based inference. It will include the model evaluation, the fisher forecast, the likelihoods, and the covariance emulators

• data_vector will be expanded to work for all type of inference, always Jaxable, with independent cosmo objects (to keep everything jaxable, the cosmo computation will be pre-computed and interpolated).

• power spectrum module will stay like that as it is generally used in all methods

• A new submodule will contain the interface with some evolving particles algorithms (we aim for flowpm for example).

This aims to include a large variety of field level methods so that flip will gain its final form, working with different methods on all possible fields