Surface area

[zachwick]

This set of commits adds a Facet.area property and a Solid.surface_area property.

[apparentlymart]

Thanks for this!

I merged a slightly-simplified version of this as ea1090fd2d5da1f432892173817e8d9617973d8f. The main change I made was to make Solid.surface_area a property that gets calculated on the fly, rather than a value that gets updated when facets change. I did this for two reasons:

• It seemed like there were lots of opportunities for the solid surface area to get out of sync with the facets, and no way to recover. e.g. the caller could explicitly set a surface area on the constructor, or a caller could call calc_surface_area a second time and double-up the surface area.
• The primary purpose of this library is to manipulate the file formats themselves, rather than to do geometry calculations. I imagine having these functions here will be handy for some folks doing quick manipulations of STL files, I'd rather only pay the cost of calculating the area when the user explicitly asks for it.

Otherwise the logic here is what was in your patch, with some simple tests I added.