Refactor plotting behavior and grid/plot saving routines

[amoodie]

Refactor plotting according to #28.

defaults

I went with defaults all false:

save_eta_figs: False
save_stage_figs: False
save_depth_figs: False
save_discharge_figs: False
save_velocity_figs: False
save_eta_grids: False
save_stage_grids: False
save_depth_grids: False
save_discharge_grids: False
save_velocity_grids: False

sequential saving

I went with sequential saving by default (eta_00000.png, eta_00001.png), and there is now a flag called save_figs_sequential that takes a boolean. Setting this to False gives eta_latest.png which is repeatedly overwritten.

refactoring

• wrap each of the make_figure and save_figure operations into functions
• remove color bar limit from eta plotting
• add variable name and timestep as title of plots
• added a bunch of tests for grid and plot saving

[elbeejay]

Looks good to me.

[ericbarefoot]

This looks great. I'm stoked that you tackled the plotting and files. That was a thorn bush I was dreading, and thus ignored. I'll go ahead and merge.