Add argument parsing for triangle/circumcircle to improve coverage

[dgtized]

Handles receiving a triangle, a map with points a,b,c and a sequential tuple of three elements.

Also adds tests to cover these cases.

This allows constructions like (map t/circumcircle triangles) to work as expected instead of throwing errors around missing keys a,b,c.

However on the API design front, I'm a little hesitant about the use of instance?. I wonder if thi.ng.geom.core needs a ICircumcircle protocol, and not just the protocol with a bounding circle? It's also not clear how to handle this if the argument is a Triangle3 at this time?