added beam file writing functionality

Hi Indranil,

Here are two functions that I use in my work to write zemax beam files. They're not technically accessing the ZDDE, but they may be useful to others.
In Python 3, there is no xrange .... so we need to do something like this:
```python
import pyzdde.config as _config
_global_pyver3 = _config._global_pyver3

if _global_pyver3:
   xrange = range
```

I can also do that after merging....