Facilitate config reset

**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