vdata_id	IN:	Vdata identifier returned by VSattach
	field_name_list	IN:	List of field names

	Purpose	Checks to see if certain fields exist in the current vdata.
	Return value	Returns a value of 1 if all field(s) exist and FAIL (or -1) otherwise.
	Description	VSfexist checks if all fields with the names specified in the parameter field_name_list exist in the vdata identified by the parameter vdata_id.
		The parameter field_name_list is a string of comma-separated fieldnames (e.g., "PX,PY,PZ" in C and 'PX,PY,PZ' in Fortran).

	FORTRAN	integer function vsfex(vdata_id, field_name_list)
		integer vdata_id
		character*(*) field_name_list