Adding `parsing` classes.

Adding `UnorderedDivergenceModelResults` and
`UnorderedDivergenceModelSummary` classes to `parsing` module.  These
classes parse and summarize the information contained with
'div-model-results' files of unordered model results. The classes will
also summarize for ordered models, but must parse the 'posterior-sample'
file instead.

Adding tests of new classes to `test_parsing.py`.

Also, updating `spreadsheet_iter` function so that it can handle (and
skip) empty lines.