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
```
Maybe we can call it UserFunction, or FunctionWrapper. We'll use it for Python in the Python wrappers, as you did, but in the (future) Julia wrappers one can use it for Julia.