Allow the use of parameters for external function

Currently external function feature can not use custom parameters as only self is allowed.

It would be interesting to have custom parameters.

The simplest way to do this is to create a dictionary parameter in the configuration file:

external_features:{
   custom_parameters:{
       "params_1": "value1",
       "params_2": 10,
       "params_3": False
   }
}

Then this dictionary should be used as input when the partial function is instanciated

The main drawback here is that all functions must accept this parameter, which is the same for all functions.

Another solution could be that the parameter refers to a file. Then this file must be loaded by the custom function directly.

In all cases, the check of these inputs must be handled in the external function as there is no way to know the type and the usage of them.