Feature/msg value descriptions

[ss32]

Adds signal values to the parsed messages.

Example output of read_dbc on current master - with the addition of <string> to common_defs.hpp:

Found 9 bus nodes.
Found 216 messages.
Found 328 signals.
Found 0 comments (Bus nodes: 0, Messages: 0, Signals: 0).
Found 1 attribute definitions (Bus nodes: 0, Messages: 1, Signals: 0).
Found 1 attribute default values.

And on this branch

Found 9 bus nodes.
Found 216 messages.
Found 328 signals.
Found 792 signal values.
Found 0 comments (Bus nodes: 0, Messages: 0, Signals: 0).
Found 1 attribute definitions (Bus nodes: 0, Messages: 1, Signals: 0).
Found 1 attribute default values.