update to plotting code: add clear() function and replace() function

[peendebak]

The clear() function to MatPlot provides the same functionality as the one in QtPlot. The replace() function is a convenience wrapper for clear(); add(...);

[alexcjohnson]

do you really want to empty traces and subplots before raising the NotImplementedError? Perhaps these should just be "example" comments?

[alexcjohnson]

@peendebak another useful add, thanks! I did a little style cleanup, including cutting down to only one place we specify the subplots and figsize defaults. I considered at the same time having a clear call that doesn't specify subplots and figsize keep the previous values of these, rather than reverting them to default. I didn't make that change though, as I could see arguments both ways... so aside from my one small comment, this looks good 💃

[peendebak]

Thanks, I added a simple explanation to the base clear() fuction, will merge the PR.