Added formula mcgrid and updated sherpa to v2.1.1 and lhapdf to v6.1.3

* Added a formula for the `mcgrid` package, which interfaces Monte-Carlo event generators with the `applgrid` package.
* Updated `sherpa` to v2.1.0. I have pinpointed the issue #15 with the event generation from the test to the fragmentation phase and turned it off for now. See the caveats section.