deprecate netket.variational in favour of netket/vqs

Created by: PhilipVinc

also deprecate netket.optim properly

netket on  deprecate [$»+] via netket_env via 🐍 system 
➜ python                                                                                              
Python 3.8.10 (default, May  4 2021, 00:00:00) 
[GCC 11.1.1 20210428 (Red Hat 11.1.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import netket as nk
nk>>> nk.variational.MCState
/home/filippovicentini/Dropbox/Ricerca/Codes/Python/netket/netket/variational.py:29: FutureWarning: The `netket.variational` module is deprecated. Use `netket.vqs` instead.
To fix this warning, change `netket.variational.MCState to `netket.vqs.MCState`.
  _warn_deprecation(
<class 'netket.vqs.mc_state.MCState'>