Add n_tables accessor to Hashbits

[kdm9]

Companion to #879

This adds khmer::Hashbits::n_tables() as a convenience method.

[kdm9]

• Is it mergeable?
• Did it pass the tests?
• If it introduces new functionality in scripts/ is it tested?
  Check for code coverage with make clean diff-cover
• Is it well formatted? Look at make pep8, make diff_pylint_report,
  make cppcheck, and make doc output. Use make format and manual
  fixing as needed.
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Is it documented in the ChangeLog?
  http://en.wikipedia.org/wiki/Changelog#Format
• Was a spellchecker run on the source code and documentation after
  changes were made?

[kdm9]

@mr-c: Any desire for these to appear in python-land? I could add that to this PR

[kdm9]

retest this please

[mr-c]

@kdmurray91 Naw, another day. Thanks!