SelectFromModel

I was trying to set multiple estimators in the selectFromModel.

```
'sklearn.feature_selection.SelectFromModel': {
'threshold': ['mean', 'median', '0.5*mean', '1.5*mean', '2*mean'],
'estimator': [ { 'sklearn model 1': { ... params .... } }, { 'sklearn model 2': { ... params .... } } ]
},
```

But TPOT complains that the estimator does not have a get_params() function.

Is it possible to have multiple estimators for the SeletFromModel option?