Convert a binary string into a decimal number
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 Syntax

        GT_BtoD(<cBin>) --> <nDec>

 Arguments:

      <cBin>   is a binary number held as a character string.

 Returns:

      <nDec>   is a decimal integer

 Description:

      This function takes a stream of '1's and '0's representing a binary
      number, and converts them to a decimal.

 Examples:

      ? GT_BtoD('11111111')        // 255

      ? GT_BtoD('1011')            // 11

 Source: GTMATH03.PRG

 Author:  Philip Ide