VALID_NUM

     Check if a string is a valid number representation.

     The input string is parsed for characters that may possibly
     form a valid number.  It is more robust than simply checking
     for an IDL conversion error because that allows strings such
     as '22.3qwert' to be returned as the valid number 22.3
     See also the original NUM_CHK which returns the status in
     the opposite sense.

     IDL> status = valid_num(string  [,value]  [,/integer])
 Inputs      : string  -  the string to be tested
 Opt. Inputs : None
 Outputs     : The function returns 1 for valid, 0 for invalid number
 Opt. Outputs: value   - The value the string decodes to.  This will be
                         returned as a double precision number unless /INTEGER
                         is present, in which case a long integer is returned.
 Keywords    : Integer   -  if present code checks specifically for an integer.
 Calls       : None
 Restrictions: None
 Category    : Utilities, Numerical
 Prev. Hist. : Small changes from NUM_CHK by Andrew Bowen,
                                             Tessella Support Services, 8/3/93
 Written     : CDS version by C D Pike, RAL, 24-May-93
 Modified    : Version 1, C D Pike, RAL, 24-May-93
               Version 2, William Thompson, GSFC, 14 October 1994
                       Added optional output parameter VALUE to allow
                       VALID_NUM to replace STRNUMBER in FITS routines.
 Version     : Version 1  24-May-93
       Converted to IDL V5.0   W. Landsman   September 1997

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