Set default text size for ATLAS labels and axis labels to 18

[mswiatlo]

Thanks a lot for this package, I've been getting used to it and like it quite a lot.

One quick suggestion: while it's not a formal statement in the style guide, 99% of ATLAS papers apply a larger axis label size, and ATLAS label size. See for example https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2018-06/fig_03b.png

I think just setting the fontsize to 18 roughly takes care of this. Note you may need to adjust the vertical spacing in the 'desc' feature of the ATLAS label function.

[beojan]

Having taken a look at that figure, I think the way to do this is to reduce the figure size, since it also has thicker lines and larger markers. I can add a large_fonts option to use_atlas_style.