Facilitate config reset

[HugoPerrier]

Description of Problem: A common error consists in using

from melusine import config
config.reset("path/to/config")

instead of :

from melusine import config
config.reset(config_path="path/to/config")

Overview of the Solution: We could raise an error if a config_path is given instead of a config dict.

Definition of Done: Config reset is simplified