Feature request: general `grade_this_plot()` type function

[garrettgman]

tblcheck::grade_this_table() and its related variants has been a phenomenally useful grading function that has become a go-to function among exercise writers. It performs a thorough comparison between the result of the students code and the result of the solution code, checking the various important ways that two tables can be different.

Can we provide something similar for grading plots?

grade_this_plot() could begin by performing a few rudimentary checks that we know work well. This would allow exercise writers to quickly begin writing code with grade_this_plot(). Over time, we could add more and better checks—and these would automatically be applied to exercises that use grade_this_plot() without the exercise writers needing to revisit the templates (and template tests should keep us from breaking anything as we do).