Fix mpi coverage in input_iterator

[grantmerz]

Problem & Solution Description (including issue #)

This adds a check in a RailStage input_iterator if mpi is not set up. Previously, an AttributeError would be raised here as self.comm would be None. Issue #116

Code Quality

• [ ] My code follows the code style of this project
• [ ] I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• [ ] My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• [ ] Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings