Minor plotting modifications

[colleenjg]

Issue 1: When plotting the shaded area in mountain_plot() using fill_between(), very large polycollections may be created (given thousands of time points). This can cause problems (i.e., the fill_between() object corrupting an illustrator file).
=> Solution: Enabled a skiprate for plotting shade in mountain_plot() using fill_between(). Allows the user to reduce the resolution of the shaded area.

Issue 2: Wall thickness cannot be adjusted for different subplot sizes.
=> Solution: Added a wall_lw keyword argument to Environment.plot_environment() to allow users to control the linewidth of the walls.

[TomGeorge1234]

Hey!

wall_lw

Would it make sense for wall_lw to go inside the **kwargs and then have a line like:
wall_lw = kwargs.get("wall_lw", 4.0)
I've used this a lot in RiaB for figure plotting params (e.g. see here) which I feel are just below the threshold of importance to warrant a dedicated kwarg, but can give the nerds access if they really want.

shade_skiprate

Thats an annoying bug I've always just put up with, and a smart solution. Again, could possible be a hidden kwarg, what do you think?

[colleenjg]

Sounds good to me! Will go implement that.

[TomGeorge1234]

awesome, let's ship it!