Add Support for PacketTable Append

Created by: TacoVox

As you guys stated on your front page, the PT interface was/is broken. With a colleague, I went ahead and improved to things:

• Removed H5Pcreate_fl and added the H5Pcreate as this is marked as deprecated by the official HDF5 documentation. As a matter of fact, I faced various issues, when trying to use variable lenght strings with H5Pcreate_fl.
• Fixed the Append function. There were countless issues with pointers and with the correct value representation in C that were fixed. I think the code will speak for itself.

Please consider this pull request.