Fixed point_source_id issues in python/liblas/{point.py,core.py}

Minor changes to what was probably intended.
See [How to get what is stored in 'data' field of las point using liblas?](http://stackoverflow.com/questions/20104139/how-to-get-what-is-stored-in-data-field-of-las-point-using-liblas) on stackoverflow for discussion of issues and fix, etc.