Random spatial sampling

[grantbuster]

Need the ability to take each batch from a different spatial location. One option is to use the current framework with a much larger spatial raster like 1e4x1e4, load all of that data into memory, and then sample batches from a random starting target.

Questions:

1. How large of a spatial "source" raster can we fit into memory with ~6 years? We should probably enable interval sampling on the timeseries data to allow for a larger dataset in mem.
2. How do we decide how many observations are in an epoch then? Kind of arbitrary since there will be a bajillion possible starting targets.

[grantbuster]

closed in #14