BUG: pytables not writing rows where all-nan in a part of a block

TST: more tests with all-nan rows/columns

DOC: corrections, add warning about dropping all-nan rows
Merge pull request #3013 from jreback/pytables_3012

BUG: pytables not writing rows where all-nan in a part of a block
CLN: eliminate assert in favor of AssertionError GH2057