[BUG] .mlvtools configuration file should be validated lazily

[SdgJlbl]

If a .mlvtools configuration file is present, the current behaviour is that any command will first check that the configuration file is correctly formatted and validate that all the paths exist, even if some part of the configuration will never be used by the command (eg the file for dvc commands when you only use ipynb_to_python, or if you provide explicitly an output path to the command). It would be best to validate the paths lazily when there are used, and to allow for incomplete configuration files if one does not wish to use all MLV-tools commands.