twStr2Num( cValue )
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     The functions twDec2Sci(), twSci2Dec(), twStr2Num() and
     twNum2Str() attempt to provide a temporary solution to Clipper's
     inability to display floating point numbers.

     twStr2Num() attempts to return a numeric value for an string
     representation of a number in the range 10-300 to 10300 when it
     is formatted with either STR() or twDec2Sci(). IF in STR() format,
     VAL() is used. If in scientific notation, twSci2Dec() is used.

     NOTE: Numbers outside the MSC calculation range will result in a
           MSC error which cannot be trapped with ERRORSYS.

       Arguments:

     cValue - 'C' String representation of a number formatted with
                  either STR() or twDec2Sci().

       Return:

     Numeric value or crash if out of range of MSC.

       Example:

     twStr2Num( "345.56" )   -> 345.56

     twStr2Num( "2.3456E12") ->  2.3456 * 1012