Algorithms for solving Coupled Differential System.

This method is used for solving Coupled Differntial System.
Given a differntial field K of characterstic 0 and f1, f2, g1, g2
in K, it decides whether the system of equations
```
     /     \        /          \    /    \       /    \
    |  Dy1  |      |  f1   af2  |  |  y1  |     |  g1  |
    |       |  +   |            |  |      |  =  |      |
    |  Dy2  |      |  f2   f1   |  |  y2  |     |  g2  |
     \     /        \          /    \    /       \    /
```
Hence returning (y1, y2) if a solution exist, None otherwise

Status : still needs to be updated

- [ ] Fix pyflakes warnings and errors
- [ ] Remove `from __future__ import with_statement`
- [ ] raise `NonElementaryIntegralException` instead of returning `None`
- [ ] Return Poly from all the functions
- [ ] Add asserts to all the tests in `test_cds.py`
- [ ] All polys in the tests should be with respect to `DE.t`