implement option to not append STATS.nc files

[emlynjdavies]

Fixes #191

In the [steps.output] step, users can now set

append = false

This is for people who want to use xarray and dask to load large datasets, which otherwise would cause substantial slowdown by appending the STATS for each raw image.

Loading a non-appended directory of nc file can be done with xarray.open_mfdataset with the below wildcards, like this:

dataset = xarray.open_mfdataset('*Image-D*-STATS.nc', combine='nested', concat_dim='index')