Improvment proposal: Add possibility to specify multiple DBC files

[erikbosch]

Could potentially be useful.

Rough possible approach:

• Keep one argument, but allow it to contain multiple files, like dbcfile=file1.dbc,file2.dbc
• If more than one, add more with https://cantools.readthedocs.io/en/latest/#cantools.database.can.Database.add_dbc_file
• If we do not have a real test case, we could possibly refactor the existing DBC file into multiple ones

[erikbosch]

Will be fixed by #113