Matrix trace
Calculates a square matrix trace.
Trace of a matrix
The trace of a square matrix is the sum of the values of its diagonal.
Let A be a square matrix of dimension n whose elements `a_(ij)`.
`1<=i<=n` and `1<=j<=n`.
`a_(ij)` being the element in line i and column j, then the trace of A is defined as,
`Tr(A) = \sum_{i=1}^n a_(ii)`