Function:   ArMax( <expA1>, [<expN1>, <expN2>] ) --> integer|float

Synopsis:   Finds the largest number in a NUMERICAL array.

            <expA1> a numerical array

            [<expN1>,
             <expN2>] optional starting and ending position for search;
                      defaults to 1 and last

Examples:   lNumbers := { 10,2789,1,0,9567 }
            lLargest := ARMax( lNumbers )   && lLargest = 9567

            lLargest := ARMax( lNumbers, 1, 4 )
                                            && lLargest = 2789

Notes:      <expN1> must always be smaller than <expN2> and mustn't
            be 0.

Returns:    If <expN1> is less than 1 or larger than <expN2>, the
            function will return NIL. Also if <expN1> is less than 1
            or <expN2> is larger than the array.
            Otherwise returns the largest number.