Grand Experiment fixes

[calebbraun]

• Parameter for the Xanthos output directory (to prevent overwriting for large runs)
• Reduce fldgen array type from float64 to float32 to keep size smaller
• 2 bugfixes

It would be better to, at some point, allow any Xanthos parameter to be passed on from the cassandra config.

We should address the issue discovered by having large arrays being passed from one node to another. The issue is that numpy arrays have an attribute nbytes which is an integer describing how many bytes are in the array. When nbytes is greater than a C int (2**31 - 1), MPI errors, as it cannot convert the number.

[rplzzz]

I opened an issue for the large arrays problem. I'm not sure whether the other item you called out is an issue for Cassandra or Xanthos.

[rplzzz]

@calebbraun Did we have anything left that we wanted to do on this one? If not, let's go ahead and make the merge.