unit management

Unit validation is now allowed at the calculation level. The
`cfunits` property of `Variable` was changed to allow `None` units
(i.e. dimensionless). Doc changes included.

Closes #183.
Update release.org
unit management #183

Added "required_units" to "AbstractUnivariateFunction" to allow for
unit validation on non-multivariate calculations. Also added a
convenience function for testing unit equality with or without
"cfunits".
Merge branch 'master' of https://github.com/NCPP/ocgis
doc changes #183
disconnect source metadata from modified metadata to allow request datasets to be reused - closes #259

The source metadata dictionary is now deepcopied prior to passing
to field. The same occurs for the variable-level metadata.
prototype integration with external climate indices library #238

Initial commit for this issue. This contains mainly changes to
the calculation interface and some new testing organization.
allow file output without auxiliary files #260

A new argument was added to "OcgOperations" called "add_auxiliary_files".
These argument will create a directory with the output contents
if True or write the file directly if False.
removed constants.fill_value

This constant was causing issues with different data types. It is
better to delegate the handling of fill values to numpy to ensure
data types are compliant when writing to netCDF4-python.
always add calendar and units attribute to time dimension for netCDF output #167

Time attributes (units and calendar) are now added to time and bounds
dimensions.

Closes #167.
method to estimate base request size #263

A method was added to "OcgOperations" called "get_base_request_size".
This method will return a dictionary estimating the requested data
size in kilobytes. Docs updated.
allow file output without auxiliary files #260

Added overwrite capability to converters. This allows env.OVERWRITE
to operate as expected without output files in an output folder.

Closes #260.
Merge branch 'master' into i249-months