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.
Is there any reason why we can't use a "real" beam file for the example, and show both functions  ``readInBeamFile()`` and ``writeBeamFile()``? Also, can the read function be named just ``readBeamFile()``?