Matrix decompositions

#### Implement Matrix Decompositions

##### TODO

- [x] LU
  - [x] fraction free version in SymPy
  - [x] normal algorithm
- [x] QR
  - [ ] Add tests after #205 is merged
- [ ] Cholesky
- [ ] Add pivoted versions of the algorithms(if applicable) after #191 is merged
I forgot to remove these commented lines. I'll remove it.