Fix crash on no feature indices

[edelangh]

If no feature indices are detected, clasifier will panic

   9:     0x56419f61951b - <core::option::Option<T>>::unwrap::hd8b8ff2812a12642
                               at /checkout/src/libcore/macros.rs:20
  10:     0x56419f61206a - rustlearn::trees::decision_tree::FeatureIndices::new::h713ea4d48acd865b
                               at /home/rico/.cargo/git/checkouts/rustlearn-1ee86889f45d433e/2ef4de2/src/trees/decision_tree.rs:58
  11:     0x56419f613345 - <rustlearn::trees::decision_tree::DecisionTree as rustlearn::traits::SupervisedModel<&'a rustlearn::array::dense::Array>>::fit::hcc2df1c85dd330cc
                               at /home/rico/.cargo/git/checkouts/rustlearn-1ee86889f45d433e/2ef4de2/src/trees/decision_tree.rs:323
  12:     0x56419f603adf - <rustlearn::ensemble::random_forest::RandomForest as rustlearn::traits::SupervisedModel<&'a rustlearn::array::dense::Array>>::fit::hba6d79b944d21aa4
                               at /home/rico/.cargo/git/checkouts/rustlearn-1ee86889f45d433e/2ef4de2/src/ensemble/random_forest.rs:119

[maciejkula]

Does this algorithm even make sense when there are no indices? Maybe it would be preferable to return an error.

[edelangh]

You're right !
It's done