Add Edelman response function

[martinvonk]

Edelman needs to be removed in future version of pastas and added to the plugins library. See Pastas issue #475

The exponential_parameters function is useful for extracting the corresponding Pastas parameters for physically realistic systems, just as kraijenhoff_parameters.

[dbrakenhoff]

The tests seem quite generic, we could just test all response functions this way instead of writing tests for each?

[martinvonk]

The tests seem quite generic, we could just test all response functions this way instead of writing tests for each?

Agree, made an issue #10