Add parsable grid data output

[spencerharmon]

The purpose of this patch is to add an option to output the grid state in JSON format each epoch along with the .mfs save files. This output includes a string with data member names and values. I've built a bit of tooling around this patch and I'm convinced it's a workable solution for easily analyzing logical (non/extra-spatial?) relationships between atoms and changes to systems of atoms within the MFM over time. There are some improvements which could be made to this feature. First, it doesn't output base layer state, which would be nice to have. Second, data member names and values are output as a non-JSON string which requires further parsing. I've made a python implementation of a parser to convert this into standard JSON: mfm-griddata-parser Third, very long strings representing data member names and values are truncated and some data is lost.