Validate input links.

def _validate_inputs(self, inputdict):
        """ Validate input links.
        """
        # Check inputdict
        try:
            parameters = inputdict.pop(self.get_linkname('parameters'))
        except KeyError:
            raise InputValidationError("No parameters specified for this "
                                       "calculation")
        if not isinstance(parameters, ParameterData):
            raise InputValidationError("parameters not of type "
                                       "ParameterData")
        # Check code
        try:
            code = inputdict.pop(self.get_linkname('code'))
        except KeyError:
            raise InputValidationError("No code specified for this "
                                       "calculation")

        # Check input files
        try:
            structure = inputdict.pop(self.get_linkname('structure'))
            if not isinstance(structure, SinglefileData):
                raise InputValidationError(
                    "structure not of type SinglefileData")
        except KeyError:
            raise InputValidationError(
                "No input structure specified for calculation")

        try:
            surface_sample = inputdict.pop(self.get_linkname('surface_sample'))
            if not isinstance(surface_sample, SinglefileData):
                raise InputValidationError(
                    "surface_sample not of type SinglefileData")
        except KeyError:
            raise InputValidationError(
                "No surface sample specified for calculation")

        # Check that nothing is left unparsed
        if inputdict:
            raise ValidationError("Unrecognized inputs: {}".format(inputdict))

        return parameters, code, structure, surface_sample