Python: Consider to remove pre-write actions

[mikir]

Consider to remove pre-write actions in Python to have write() method without any side effect.

Consider as well to extend Python runtime library by serialize() to call method initializeOffsets() automatically before serialization similarly as C++ does. This issue is a Python clone of #342.

[mikir]

The following can help to understand possibly compatibility problems in user Python applications which can be caused by this issue:

1. Everywhere where application calls zserio write() method or where application passes zserio objects to API for serialization (pubsub, service, SQL table), the application must call initialize_offsets() method explicitly.
2. Method initialize_offsets() can be called without bitposition parameter. This parameter is by default 0 now.
3. Python runtime functions serialize(), serialize_to_bytes() and serialize_to_file() call now initiliaze_offsets() automatically.
4. Python runtime function deserialize_bytes() has been renamed to deserialize_from_bytes() to be consistent.