Several functions involved in downscaling were referring to columns using indices, instead of names. This is no robust to changes in column order. As part of the changes to use column names throughout I have:

added assertions to ensure input meet requirements
added more tests to make sure that results are consistent with those produced by the latest release (head of main branch);
collapsed the functions process_one_gcm, process_one_historic and process_one_gcm_hist into a single function.
xyz and res (internal object) are now data.tables.
xyz must now be a 4 column data.table with lon, lat, elev and id columns. All other columns are ignored and NOT returned.
xyz and xyzID became identical and so xyzID is now simply xyz

This PR also addresses

bcgov / climr

Refer to columns by name, not index + using `data.table` #183

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