Also improved KnmMatrix constructor by removing need for pre-constructed buffer. By default,
an identity matrix (of complex values) is allocated in KnmMatrix.__init__ now. To make
a KnmMatrix from a pre-existing array, one can now use KnmMatrix.from_buffer.

Other minor changes include stripping whitespace from possible str keys passed to
KnmMatrix.__getitem__ and added a log option for plotting KnmMatrix objects in
amplitude mode.