allow_lambda_layers parameter not working for SmoothGrad and IntegratedGradients

Hi, I believe there may be an issue with SmoothGrad and IntegratedGradients methods when there's need to set `allow_lambda_layers=True` during the creation of the analyzer.

In particular I think that `"allow_lambda_layers"` is missing in `kwargs_keys` defined in the `__init__()` method of both classes `SmoothGrad` and `IntegratedGradient`, so it cannot be passed to the inner `Gradient` constructor for both methods.

A snippet of the code I use is the following (but the same happens for method "integrated_gradients"):
```python
analyzer = innvestigate.create_analyzer("smoothgrad", model, allow_lambda_layers=True)
analyzer.analyze(preprocessed_input)
```

while the stacktrace is:

```
Traceback (most recent call last):
File "/home/davide/PycharmProjects/innvestigate-gui/innvestigate-gui/main.py", line 890, in visualize
allow_lambda_layers=True)
File "/usr/local/lib/python3.6/dist-packages/innvestigate/analyzer/__init__.py", line 140, in create_analyzer
return analyzer_class(model, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/innvestigate/analyzer/gradient_based.py", line 310, in __init__
**kwargs)
File "/usr/local/lib/python3.6/dist-packages/innvestigate/analyzer/wrapper.py", line 219, in __init__
*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/innvestigate/analyzer/wrapper.py", line 97, in __init__
*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/innvestigate/analyzer/wrapper.py", line 49, in __init__
*args, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'allow_lambda_layers'
```

Among these 2 methods, with all the others creating the analyzer with `allow_lambda_layers=True` works correctly.

If you need any extra information let me know.
Thanks.