Rename implementation files in netket.operator

Created by: femtobit

As brought up in issue #445 (closed), it can be confusing (at least with auto-completion) if netket.operator contains both LocalOperator directly (which users are supposed to use) and the submodule local_operator.py where that class is implemented. Thus, this implementation file is renamed. And for consistency, so are all the others to which the same argument applies.

Since none of the current PRs modify any operator code, now seems to be as good as time as any to do this with minimal conflicts.