v0.3.9

This is a minor release focusing on under-the-hood improvements.

A new method, from_xarray is added for converting xarray datasets to nctoolkit datasets.

Methods for identifying how many missing values appear in datasets have been added: na_count and na_frac. These will identify the number or fraction of values that are missing values in each grid cell. The methods operate the same way as the temporal methods. So ds.na_frac("year") will give you the fraction of values are missing values each year.

Methods for better upscaling of datasets will be added: box_mean, box_sum, box_max. This will allow you to upscale to, for example, each 10 by 10 grid box using the mean of that grid box. This is useful for upscaling things like population data where you want the upscaled grid boxes to represent the entirety of the grid box, not the centre.

Improvements to merge have been made. When variables are not included in all files nctoolkit will now only merge those in each file in a multi-file dataset. Previously it threw an error.

Functions for finding the times and months in netCDF files are now available: nc_years and ``nc_months`.

The attribute variables_detailed has been changed to contents. It will also now give the number of time steps available for each variable.

cdo_command now allows users to specify whether the CDO command used is an ensemble method. Previously methods applied on a file by file basis.