Solutions for quadratic ternary forms implemented

Solutions for the ternary quadratic equation `ax^2 + by^2 + cz^2 + dxy + eyz + fyz = 0` implemented.
Currently uses the `descent()` method to find a non trivial solution.

###### Major TODOs:

- [x] Verify the results with MAGMA's algorithm
- [x] Implement Holzer Reduction
- [x] Use the function `sqrt_mod()` from Pernici's [PR](https://github.com/sympy/sympy/pull/2307/)