Adds the tangent normalization step to the NormalizeSomaticCounts tool.

- The previous target-factor normalization only output is now relegated to an optional output.
- Also had added options to output the pre-tangent normalization counts and beta-hats.
- Fixed a limitation of HDF5PoN that was not able to handle the possibility that the reduced PoN has a different set of targets as the target-factors array in the PoN file.

This addresses issue #8.