Nonparametric kernel density estimation and regression.

See #408 for history. Now renamed ``KDE`` and ``ConditionalKDE`` to ``KDEMultivariate`` and ``KDEMultivariateConditional`` respectively. Untested parts moved to sandbox.

Still TODO:

  - some test failures, mainly precision issues.
  - rename ``Reg``?
  - is run time of tests acceptable?
  - test coverage is about 75%, could be better.
  - does kernels.py need to be renamed? There's also a kernels.py in sandbox, used for the existing ``KDE``.
  - clean up nonparametric module some more? Current names for ``KDE``, ``bandwidths`` are too generic.
  - add more docs, especially a good module docstring listing and comparing all functionality.