sympy.erf does not print its arguments

Caused by a bad implementation of `GaPrinter._print_Function`:

```python
In [12]: from sympy import *

In [13]: x = Symbol('x')

In [14]: erf(x)
Out[14]: erf(x)

In [15]: import galgebra.printer

In [16]: sympy.erf(x)
Out[16]: erf
```

Hopefully can be fixed along with #206 and #276.
