Move resource validation of CalcJob from global validator to port itself

[sphuber]

The metadata.options.resources of a CalcJob are validated by a validator on the global inputs namespace of the process. This makes it difficult for sub classes to control this behavior. If a sub class wants to make the resources optional, for example to implement automatic parallelization that would set the resources automatically, the validation would always fail.

[sphuber]

A problem might be that currently the validation of the resources requires the Computer that will be used as input of the CalcJob. The spec of a validator currently only passes the value that is passed for the port itself. There is no context. So the validator of metadata.options.resources will not have access to the computer passed elsewhere in the inputs. This would require a change in the validator signature and handling of plumpy.

[sphuber]

The required change in the validator signature and calling in plumpy is discussed here: aiidateam/plumpy#126