;-------------------------------------------------------------
;+
; NAME:
; AURORAX_CREATE_METADATA_FILTER_EXPRESSION
;
; PURPOSE:
; Create metadata filter expression for searches
;
; EXPLANATION:
; The AuroraX ephemeris, data products, and conjunction searches can take
; metadata filters to help refine your search. This function provides
; an easy way to create metadata filter expressions which will be added
; to a metadata_filter object later on.
;
; CALLING SEQUENCE:
; aurorax_create_metadata_filter_expression(key,values)
;
; PARAMETERS:
; key the metadata field key, string
; values the values to filter on, string or list
;
; KEYWORDS:
; /OPERATOR_EQ operator for this expression will be '='
; /OPERATOR_NE operator for this expression will be '!='
; /OPERATOR_GT operator for this expression will be '>'
; /OPERATOR_LT operator for this expression will be '<'
; /OPERATOR_GE operator for this expression will be '>='
; /OPERATOR_LE operator for this expression will be '<='
; /OPERATOR_BETWEEN operator for this expression will be 'between'
; /OPERATOR_IN operator for this expression will be 'in'
;
; OUTPUT:
; a metadata filter expression
;
; OUTPUT TYPE:
; a hash
;
; 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)
;+
;-------------------------------------------------------------