Properly handle case for data assimilation with no data points on the mesh

[stvdwtt]

Currently during data assimilation the code will segfault if no rays hit the mesh. (Presumably the same for measurements with a specified location.) We need to decide how we want to gracefully handle this.

Options:

• Skip that data assimilation step and print out a warning (I think this is the better option)
• Abort