Aggregations should store version information on the alias list

[afmagee42]

Per conversation with some folks at CDPHE, an aggregation depends on the exact alias map that went into it. We ought to find a way to store this information usefully.

In-python, it should be an attribute of Aggregation. Outside of that... Does this mean we should institute a special save/read convention or format? Perhaps all we need is Aggregation.write(), which makes a json with the highest levels being (1) the mapping (current output of Aggregation.to_str(), and (2) the version info?

[afmagee42]

cc @swo re: file formats

[swo]

json seems good! these aren't big files, and json is a good standard