Add HDF5 map data export

[tedsteiner]

This will allow large maps to be parsed once and then reloaded as .jld files, taking up less system memory for large maps.

[tedsteiner]

Probably is better to instead put simple instructions into the documentation rather than adding another dependency.

[tedsteiner]

This seems to be about as slow as just reading the data from a file. Saving the data is straightforward, so I'm just going to skip this.