Add Maths Convert blocks

This add Math blocks that converts:-

Decimal to Hexdecimal
Hexdecimal to Decimal
Decimal to Binary
Binary to Decimal

It validates the input is correct before converting


This should signal an error, not just return the string "error".   Instead of  "error", use
        (signal-runtime-error
         (format #f "Convert hex to dec: The string ~A is not a valid hexadecimal number"  (get-display-representation x))
         "Invalid hexadecimal number"))