Updating the functionality of barycentric vectors

[nzfeng]

Nothing major -- just added additional functionality to the BarycentricVector object:

• rotated90() now supports edge-type vectors as input
• a new function rotated() allows rotation of vectors by arbitrary angles
• added some more detailed comments in the code so I don't forget how I derived quantities :)
• documentation updated accordingly.

Let me know if you need me to add additional test functions to geometry-central -- for these few commits, I tested everything using an external program: https://github.com/nzfeng/barycentric-vector-demo

[nmwsharp]

This looks great! Docs are good too. I left two small comments above, but neither are big issues, just small questions.