add file output to cli plotting

[thomaszwagerman]

Addresses #22

Adding the ability to write files using the asli_plot cli command. Adding the -y --year parameter to trigger either a.plot_region_all() or a.plot_region_year().

Generalising the common -o --output parameter description.

[thomaszwagerman]

There's a couple of design decisions to discuss:

1. Importing plt in asli.py just to call plt.savefig() (to quickly make this work). Would it be better to define a separate function in plot.py and from .plot import ...
2. Passing additional arguments plt.savefig() can take (e.g. dpi, bbox_inches, see matplotlib docs). Is there an elegant way to do this or would it be a case of writing out additional _get_cli_plot_args() arguments?

[davidwilby]

Again sorry for the delay!! 😭

1. Importing plt in asli.py just to call `plt.savefig()` (to quickly make this work). Would it be better to define a separate function in plot.py and from .plot import ...

I don't actually mind either way, ultimately plt/matplotlib gets imported so it doesn't do anything for us in terms of speed/resources, but it may be tidier to keep all lower-level plotting in the plot module. Not a big deal either way for this package I think but it's always good to consider these choices.

2. Passing additional arguments `plt.savefig()` can take (e.g. `dpi`, `bbox_inches`, see [matplotlib docs](https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.savefig.html)). Is there an elegant way to do this or would it be a case of writing out additional `_get_cli_plot_args()` arguments?

This I'm not entirely sure about, but it would be nice to give the user control of all of those params from the CLI I guess? Here follows a stream of consciousness in lieu of any actual coherent thoughts:

I kind of think that we should allow some common options (i.e. anything we need) but only give the user fine control over the plotting object by just returning it for their use.

So at the moment the plot module is just a small collection of convenience functions but maybe we should have a class or maybe as in asli.plot.plot_lows returning the axis object is sufficient? I'm not particularly au fait with matplotlib so unsure whether this would be ordinary practice.