Remove update_merge

[thodson-usgs]

update_merge() is only called by _read_json() and should be removed or improved: For example, the current implementation will fail when the method code is 'x' (#129).

I tried replacing it with a simple merge, but this breaks multisite json queries,
which return a list of timeseries elements: one for each combination of site number and parameter. Thus we cannot merge, because parameter 2 of site 2 will already be populated by NA (from merging site 2 parameter 1, with site 1; and so on and so forth).