added is_constant(sig) helper method
Passing n as kwarg to Signal constructor

Unfortunately Signal(3) does not mean a signal of constant 3.  It means
a signal of 3 dimensions. I changed one little unit test to read Signal(n=3)
but a better fix to this instance of bad readability might be in order...
new encoder/decoder pathway test in TestSimulator
new: test_encoder_decoder_with_views
Changed to assertTrue instead of bare assert