Turns a Hex string into a decimal integer
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 Syntax

        GT_HtoD(<cHex>) --> <nDec>

 Arguments:

      <cHex>  is a hexadecimal number, held as a string

 Returns:

      <nDec>  is a decimal integer

 Description:

      This function allows hex numbers to be converted into decimals.
      This allows calculations to be applied on hex values
      (e.g. incrementing).

 Examples:

      ? GT_HtoD('FF')       // 255

      ? GT_HtoD('F')+1      // 16

      ? GT_HtoD('A')*2      // 20


 Source: GTMATH04.PRG

 Author:  Philip Ide