v0.19.0

Breaking changes

• Refactoring of the Indicator class. The cfprobe method has been renamed to cfcheck and the validate
  method has been renamed to datacheck. More importantly, instantiating Indicator creates a new subclass on
  the fly and stores it in a registry, allowing users to subclass existing indicators easily. The algorithm for
  missing values is identified by its registered name, e.g. "any", "pct", etc, along with its missing_options.
• xclim now requires xarray >= 0.16, ensuring that xclim.sdba is fully functional.
• The dev requirements now include xdoctest -- a rewrite of the standard library module, doctest.
• xclim.core.locales.get_local_attrs now uses the indicator's class name instead of the indicator itself and no
  longer accepts the fill_missing keyword. Behaviour is now the same as passing False.
• Indicator.cf_attrs is now a list of dictionaries. Indicator.json puts all the metadata attributes in the key "outputs" (a list of dicts).
  All variable metadata (names in Indicator._cf_names) might be strings or lists of strings when accessed as object attributes.
• Passing doctests are now strictly enforced as a build requirement in the Travis CI testing ensemble.

New features and enhancements

• New ensembles.kkz_reduce_ensemble method to select subsets of an ensemble based on the KKZ algorithm.
• Create new Indicator Daily, Daily2D subclasses for indicators using daily input data.
• The Indicator class now supports outputing multiple indices for the same inputs.
• xclim.core.units.declare_units now works with indices outputting multiple DataArrays.
• Doctests now make use of the xdoctest_namespace in order to more easily access mdoules and tesdata.

Bug fixes

• Fix generic.fit dimension ordering. This caused errors when "time" was not the first dimension in a DataArray.

Internal changes

• datachecks.check_daily now uses xr.infer_freq.
• Indicator subclasses Tas, Tasmin, Tasmax, Pr and Streamflow now inherit from Daily.
• Indicator subclasses TasminTasmax and PrTas now inherit from Daily2D.
• Docstring style now enforced using the pydocstyle with numpy doctsring conventions.
• Doctests are now performed for all docstring Examples using xdoctest. Failing examples must be explicitly skipped otherwise build will now fail.
• Indicator methods update_attrs and format are now classmethods, attrs to update must be passed.
• Indicators definitions without an accompanying translation (presently French) will cause build failures.
• Major refactoring of the internal marchinery of Indicator to support multiple outputs.