Implement Matrix functions

* Submatrix
* Transpose
* Fraction free gaussian elimination
* Diagonal solve
* Fraction free versions of Gauss elimination and Gauss Jordan elimination
* LU decomposition
This is commented out for the time being as it doesn't produce the answers in the reference: Fraction-free algorithms for linear and polynomial equations (1997) by George Nakos, Peter Turner, Robert Williams. I'll fix it soon.