RF: Rewrite of the tensor mode calculation with no loops.

Hey @swederik - I was very excited about the idea of calculating the tensor mode, so I've done a bit of a rewrite of what you had, with no for loops. Basically, I calculate each one of the quantities that are required (determinant, norm) under the assumption of an array that has a shape that ends with (3,3). This makes it possible to do the calculation without looping over the elements. I've also added a little bit of testing and changed function names and documentation, etc. Let me know what you think!