Added
- Electrons can now accept an executor object using the "backend" keyword argument. "backend" can still take a string naming the executor module.
- Electrons and lattices no longer have Slurm metadata associated with the executor, as that information should be contained in the executor object being used as an input argument.
- The "backend" keyword can still be a string specifying the executor module, but only if the executor doesn't need any metadata.
- Executor plugin classes are now directly available to covalent, eg: covalent.executor.LocalExecutor().