add tet tiling tip

Library/tips.json

 {"tips":[
+"Regular tetrahedra do not tile <i>R<sup>3</sup></i>.",
 "Splitting the faces of a cube into pairs of rectangles of alternating orientation yields a dodecahedron.",
 "A triangle's centroid, circumcenter, and orthocenter are collinear.",
 "The Heat Kernel Signature is scale-sensitive.",