[Docs] how to export high resolution TIFF images

[lisphilar]

As asked by @AnujTiwari with #851, how to export high resolution TIFF images is not documented currently.

Meanwhile, please let me know how to export the intermediate maps (snl.trend().summary()) in high resolution .tif image format. Thank you

When we use CovsirPhy with Jupyter Notebook, we can export TIFF files as follows.

snl.interactive = False
snl.trend(filename="image_filename.tiff", dpi=300)

When we use it with Python scripts, snl.interactive = False is un-necessary.

snl.trend() (and all methods which create figures) calls matplotlib.pyplot.savefig() internally. Keyword arguments of matplotlib.pyplot.savefig(), including dpi=300, can be used as the keyword arguments of snl.trend(). If not, that may be a bug.