Metadata ignore subset columns

[olgabot]

Add option to ignore columns in the metadata for creating sample subsets.

• Is it mergable?
• Did it pass the tests?
• If it introduces new functionality in is it tested?
  Check for code coverage. To run code coverage on only the file you changed,
  for example flotilla/compute/splicing.py, use this command:
  py.test --cov flotilla/compute/splicing.py --cov-report term-missing flotilla/test/compute/test_splicing.py
  which will show you which lines aren't covered by the tests.
• Do the new functions have descriptive
  numpydoc
  style docstrings?
• If it adds a new feature, is it documented as an IPython Notebook in
  examples/, and is that notebook added to doc/tutorial.rst?
• If it adds a new plot, is it documented in the gallery, as a .py file
  in examples/?
• Is it well formatted? Look at make pep8 and make lint output
• Is it documented in the doc/releases/?
• Was a spellchecker run on the source code and documentation after
  changes were made?

[coveralls]

Coverage increased (+0.01%) to 53.58% when pulling 7aa9053 on metadata_ignore_subset_columns into 6ab59c1 on master.

[coveralls]

Coverage increased (+0.04%) to 53.61% when pulling 7aa9053 on metadata_ignore_subset_columns into 6ab59c1 on master.