This adds an option to the differential encoder an decoder blocks to perform NRZI encoding and decoding. NRZI only makes sense with a modulus of 2, so the blocks constructors will throw and exception if passed nrzi = true and a modulus different from 2. The GRC blocks handle this by hiding the modulus field if the user selects NRZI encoding. A new unit test for the NRZI version of the blocks is added. Besides checking that encode plus decode gives the original, this test also compares the C++ implementation results against a Numpy implementation. Additionally, a faster implementation of differential encoding/ decoding for modulus 2 is included here. Signed-off-by: Daniel Estévez <daniel@destevez.net>