Sympy object pointer in CSymPy

```
In [1]: from csympy.lib.csympy_wrapper import *

In [2]: import sympy

In [3]: sympy.var("x y z w")

In [4]: a = Integer(1) + sympy.Ynm(x,y,z,w)

In [5]: type(a)
Out[5]: csympy.lib.csympy_wrapper.Add

In [6]: b = a.args[1]

In [7]: type(b)
Out[7]: csympy.lib.csympy_wrapper.SympyFunction

In [8]: c = b._sympy_()

In [9]: type(c)
Out[9]: sympy.functions.special.spherical_harmonics.Ynm
```
With `void *`, we can then also test it in C++ by simply creating a user defined class (at runtime) and plugging it in.