Enable sphinx nitpicky on doc builds

[dopplershift]

Sphinx has a nitpicky mode that warns about missing references. This seems like something we would naturally be interested in. This can be enabled by adding to docs/conf.py:

nitpicky = True

Currently, this spews a ton of warnings that I'm not exactly sure how to fix. Here's just a sample from the end:

/Users/rmay/repos/metpy/metpy/units.py:docstring of metpy.units.pandas_dataframe_to_unit_arrays:4: WARNING: py:obj reference target not found: pandas.DataFrame
/Users/rmay/repos/metpy/docs/examples/gridding/Natural_Neighbor_Verification.rst:130: WARNING: py:func reference target not found: metpy.interpolate.geometry.find_natural_neighbors
/Users/rmay/repos/metpy/docs/units.rst:4: WARNING: py:mod reference target not found: pint
/Users/rmay/repos/metpy/docs/units.rst:34: WARNING: py:class reference target not found: pint.Quantity
/Users/rmay/repos/metpy/docs/units.rst:100: WARNING: py:class reference target not found: pint.Quantity
/Users/rmay/repos/metpy/docs/units.rst:100: WARNING: py:meth reference target not found: pint.Quantity.to
/Users/rmay/repos/metpy/docs/units.rst:100: WARNING: py:class reference target not found: pint.Quantity
/Users/rmay/repos/metpy/docs/units.rst:111: WARNING: py:meth reference target not found: pint.Quantity.ito
/Users/rmay/repos/metpy/docs/units.rst:111: WARNING: py:meth reference target not found: pint.Quantity.to_base_units
/Users/rmay/repos/metpy/docs/units.rst:124: WARNING: py:meth reference target not found: pint.Quantity.to_base_units
/Users/rmay/repos/metpy/docs/units.rst:124: WARNING: py:meth reference target not found: pint.Quantity.ito_base_units

I feel like these should be corrected if they're not resolving.