// This function creates an index according to the value of its input
// (sorts by Right Ascension, Field name or WFC data status)
// If sorting by Field name, use alphabetical order. Fields that start with
// numerical characters come before letters.
// If sorting by RA, use 1st four characters (hh mm) to create integer between
// 0000 and 2359, and sort by increasing RA. If 2 fields have the same use 
// field name.
// If sorting by WFC status, give 1 per band with data exisiting - gives 
// integer between 0 and 5. Then give 1 if merged data, 0 if not
// Inputs:
// $sort: what to sort on
// $ra: string array with ra of field centre in hh mm ss.s
// $field: string array with field names
// $wfc: multidimensional array containing info about wfc data
//
// Written by Suzanne Aigrain on 05-Jan-2001
// Modified on 07-Feb-2001 to sort by increasing RA

(See /home/xmmssc/soft/php/srt_fields.php)