Added decoder.py to RegisterMachine, provides inverse operations to Encoder.

Added decode_pair: provides a pair representation of a numerical representation.
Added tests for Decoder.decode_pair
Added tests using both Encode and Decode one after another.
Added decode_list: returns a list for a numeric representation, inverse of Encoder.encode_list()
Added tests for Decoder.decode_list()
Bugfixs in Decoder.decode_list()
Added encode decode tests for encode/decode list
Added further tests for Decode.decode_list