added station_selector() and var_selector() to handle specifications of station lists (explicit list or region lon/lat) and variables
fixed buts in converting variable ids to json equivalents in list_raws_in_bbox and modified extract_observations_for_var to skip any non-numeric values returned from mesowest json api
added acquire_observations/3 function to retrieve observations on-demand
retrieve_unit -> varid_to_unit and xform_value now throws an exception for unknown variable ids or for non-numeric values
added support for station selectors to app startup
modified app file so that all stations in Colorado are queried for observations
unified api retrieve_observations/3 to use station_selector() and var_selector() types, added acquire_observations/3 which retrieves observations from mesowest
added basic type checking to raws_ingest API functions
fixed reply tuple in clear_errors in raws_ingest_server:handle_call