Added 6 new functions and made minor fixes.

1. New functions added
zGetFile, zGetFirst, zGetSurfaceData, zGetSurfaceDLL,
zGetSurfaceParameter, zSetSurfaceParameter

2. Added check for validity of input for function zGetTextFile(). This
will prevent Zemax DDE <-> Python communication from stalling if the
file path is not full-path or if it doesn't have any extension.

3. Moved file-end notes to unit-test file in-order to keep this file
clean.
Added unit-test for newly added functions.