Create a FluxCore package?

[CarloLucibello]

Since Flux doesn't impose a type hierarchy, any model/layer can be defined in a Flux-unaware matter and then be used in combination with Flux.

Nonetheless, we can factor out some basic functionalities that are convenient to have without relying on the whole Flux.
This is the motivation for creating a FluxCore package.
It could contain the following:

• @layer
• state
• loadmodel!
• .... what else?

As an example, with these 3 methods Tsunami.jl could remove the Flux dependence in favor of a FluxCore one.

For convenience, the FluxCore.jl package could be hosted in this repo.