Multiple edits

biom_calc.py:
- updated docstrings for all functions.
- updated raw_abundance() and added transform_raw_abundance() function.

test_biom_calc.py:
- updated tests for raw_abundance() and added test for transform_raw_abundance().

otu_calc.py:
- updated docstrings for all functions.
- deleted calculate_total_abundance(), biom_summary() and relative abundance()
- updated assign_otu_membership(), which now calls relative_abundance() from util.py

test_otu_calc.py:
- created automated testing file for otu_calc.

util.py:
- updated docstrings for all functions.
- added gather_categories() and parse_unifrac() functions.

test_util.py:
- added test_gather_categories() and test_parse_unifrac() tests.