Add a plotting style with bigger labels

Created by: Zaharid

The default label sizes are intended for figures that occupy all of the page, but we often want to have them at two or three columns. In that case the plotting text is typically way too small. Add a style file that makes them bigger.

At the moment one needs to run with

validphys --style <path/to>/biglabels.mplstyle

but I'll make a separate change to reportengine and vp to make that simpler.