Implement persistence images

[rrrlw]

Introduced in this JMLR article

[corybrunson]

Something to consider for the broader tdaverse is a standalone package for vectorizations of persistence data. These might include

• persistence images
• persistence landscapes (though these might warrant a standalone package)
• persistence rank function
• @rrrlw your own use of dimension-wise first moments

I mention this because it would save {ggtda} a lot of code without introducing dependencies; the user would generate the vectorizations externally, but then {ggtda} would know how to visualize them.

Once done, it would be quick and valuable to provide a {recipes} extension for these vectorizations. Building a recipes extension is like building a {ggplot2} extension, only more so. I made a small one for association rules, and {embed} is a great point of reference.

[corybrunson]

Subsumed into #54.