Add space complexity metrics

[frneer]

This patch adds the code to compute space complexity metrics. Supporting both UniformQuantizer and FlexQuantizer.

For the FlexQuantizer, we introduce the nominal huffman complexity computation, to get a relative value of the expected size when implementing that in hardware.

[frneer]

@colorete87 PTAL, see the tests. I also tested with some examples locally and it seems to be working. We still need to add tests for huffman computation, but that will require manually computing the results and creating the tests accordingly.

[colorete87]

When running the test with $ ./metrics_test.py -k test_flex_quantizer_space_complexity a strange behaiour is observed: the computed complexity (computed_bits) sometimes is zero and sometimes don't:

• Example with non-zero result:
  
• Example with complexity equal to zero: