Add matplotlib stylesheets

By popular request.

Use matplotlib 3.7's third aprty package style feature: https://matplotlib.org/stable/users/prev_whats_new/whats_new_3.7.0.html#style-files-can-be-imported-from-third-party-packages.

To Do

• Enforce defaults on all other parameters so that a given style is reproducible. One can always use multiple styles if necessary. Users should use with plt.style.context(style, after_reset=True): ... for this.
• Move styles to qutil/plotting
• Add function qutil.plotting.list_styles()