ORM redesign: rename JobCalculation to CalcJobNode

[sphuber]

With the new hierarchy in place, it is probably best to already move the implementation for the old JobCalculation class there as well, even though we might want to keep an alias to JobCalculation for the time being, to not break plugins too heavily. However, doing this, will make the future migration of separating the node from the process a lot easier and it allows us to delete the old aiida.orm.calculation hierarchy.