refac: build on LuxCore

[MartinuzziFrancesco]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

This PR refactors ReservoirComputing.jl to build on LuxCore. Closes #206, #202

[MartinuzziFrancesco]

Some naming things to decide:

• signature fro train! either
  • train!(rc, train_data, target_data, ps, st, sr::StandardRidge=StandardRidge(0.0)) (current)
  • train!(rc, train_data, target_data, sr::StandardRidge=StandardRidge(0.0), ps, st)
• name of the collect states function (there a problem with the states nomenclature too, considering Lux as st as states)
  • collectstates (current)
  • collect_reservoirstates
  • collect_reservoir_states
• collection layer
  • Collect() (current)
  • CollectLayer()
• readout layer
  • Readout() (current)
  • LinearReadout() even if it could be not linear, this reflects that it is obtained through linear regression, and I actaully like it more at the moment
• train! or fit!?