Add the CSymPy package

It installs and works in Python. At least on linux. Here is how to try it out:
```
In [1]: from csympy import var

In [2]: var("x y z")
Out[2]: (x, y, z)

In [3]: e = ((x+y+z+1)**15+1)*(x+y+z+1)**15

In [4]: e
Out[4]: (1 + (1 + y + x + z)^15)*(1 + y + x + z)^15

In [5]: %time f = e.expand()
CPU times: user 1.43 s, sys: 4 ms, total: 1.44 s
Wall time: 1.43 s
```