Weird behaviour for the `title` header field

[zacharyrs]

Hi,

It seems mmtf-python has a bit of a bug when it comes to the optional title field in the header.
Specifically, see lines 103-107 of mmtf_reader.py.

Firstly, it seems the sys version check does nothing?
Secondly, I think title should default to None, given it is an optional field, like how rFree is handled.
Currently this causes a crash in decoder_utils.py for files without the title field.