Solving a system of linear diophantine equations

Find the basis of solutions as vectors in natural numbers using the algorithm
presented [here](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.47.2737). The current implementation of the solutions of linear diophantine 
equations in diophantine module should be removed after this is merged since it is not 
complete.

###### TODO
- [ ] Implement the solutions for inhomogeneous case
- [ ] Add more tests
- [ ] Find the solutions in negative integers if possible