aurorax_create_criteria_block
;-------------------------------------------------------------
;+
; NAME:
; AURORAX_CREATE_CRITERIA_BLOCK
;
; PURPOSE:
; Create conjunction search criteria block
;
; EXPLANATION:
; The AuroraX conjunction search takes in sets of ground, space,
; and/or events "criteria blocks". These are objects describing
; the items to search for conjunction between. This function is
; meant to be an easy way to create a criteria block object
; for use in a subsequent conjunction search.
;
; CALLING SEQUENCE:
; aurorax_create_criteria_block
;
; PARAMETERS:
; programs programs for this criteria block, list(string), optional
; platforms platforms for this criteria block, list(string), optional
; instrument_types instrument types for this criteria block, list(string), optional
; hemisphere hemisphere values for this criteria block, list(string),
; optional (valid values are 'northern' and/or 'southern')
; metadata_filters metadata filters to filter for, hash, optional
;
; KEYWORDS:
; /GROUND create a "ground" criteria block
; /SPACE create a "space" criteria block
; /EVENTS create a "events" criteria block
;
; OUTPUT:
; the criteria block
;
; OUTPUT TYPE:
; a struct
;
; EXAMPLES:
; expression = aurorax_create_metadata_filter_expression('calgary_apa_ml_v1',list('classified as APA'),/OPERATOR_IN)
; expression = aurorax_create_metadata_filter_expression('calgary_apa_ml_v1_confidence',95,/OPERATOR_GE)
; expression = aurorax_create_metadata_filter_expression('tii_on','true',/OPERATOR_IN)
; expression = aurorax_create_metadata_filter_expression('tii_quality_vixh','0,2',/OPERATOR_BETWEEN)
;+
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