DBC files: Enable utils.py to parse DBC files in a Bus Channel specific manner

[MatinF]

Currently DBC files are applied across the entire log file. An improvement could be to enable parsing a DBC object ala below:

dbc_paths = {"CAN": [("dbc_files/CSS-Electronics-SAE-J1939-DEMO.dbc", 0)]]}

Here, 0 would refer to the DBC being applied across all channels, 1 would mean only for Bus Channel 1, 2 would mean only for Bus Channel 2.

This requires a modification to the logic in load_dbc_files and extract_phys.