Fixed bug in 'mean_free_path' option.

It was impossible to enter a numerical value.
The 'allowed_values' keyword was set in the definition
of the 'mean_free_path' parameter, I commented it to
allow any value for this option.

src/msspec/parameters.py

@@ -1539,7 +1539,7 @@ class CalculationParameters(BaseParameters):
             Parameter('cutoff_factor', types=(int, float),
                       limits=(1e-4, 999.9999), default=0.01, private=False),
             Parameter('mean_free_path', types=(int, float, str),
-                      default='SeahDench', allowed_values=('mono', 'SeahDench'),
+                      default='SeahDench', #allowed_values=('mono', 'SeahDench'),
                       doc="""
             The electron mean free path value. You can either:
             - Enter a value (in Angströms), in this case any value <=0 will disable the damping