add support for container-pdus without sub-pdu Header

[tainnok]

!! DRAFT !!
this is part of an bigger change which i tried to split into several pull-requests.
But as the changes depends on each other the splited pull-requests will generate merge-conflicts.
So see this seperated pull-requests as some kind of documentation.
The "big" pull-request which contains everything in a single pull-request which do NOT generate merge-conflicts is: #873

WARNING: most likley #864 is merged before this PR.
You will get an merge-conflict at the call of get_signals() in get_frame_from_container_ipdu().
The version of this PR need to be choosen:
get_signals(pdu_sig_mapping, target_pdu, ea, None, float_factory, bit_offset=0, generated_update_bits_init_to_1=generated_update_bits_init_to_1)

there are two kinds of container-pdus.

• dynamic, where each sub-pdu has an header (id and length). not all sub-pdus are transmitted each time, position of sub-pdu in container-pdu is dynamic
• static, where each sub-pdu has a fixed position (offset) in the container-pdu. there is NO header for the sub-pdus!

there was some kind of support for static container-pdus, but in that case each signal of a static sub-pdu got modified start_bit (increased by the pdu-offset).
That result in a situation where the start_bit was incorrect in relation to the sub-pdu itself.
As the sub-pdu offset was not stored, the user of canmatrix has no possiblity to get the correct signal-startbit positions.

Solution:

• store the sub-pdu offset at the sub-pdu itself
• set the startbit of the sub-pdu signals to the correct value
• add support for no-header container-pdus to the unpack() method