Change types for some TreeBoosterParameters to match types in XGBoost

[KyussCaesar]

While working on a personal project using this repo I noticed that some of the types for the parameters in TreeBoosterParameters don't seem to match up with what's in XGBoost? You can check them out here.

Changelog

• Change types for a few TreeBoosterParameters to match types in XGBoost:
  • gamma: u32 -> f32
  • min_child_weight: u32 -> f32
  • max_delta_step: u32 -> f32
  • lambda: u32 -> f32
  • alpha: u32 -> f32
• Add some missing parameters:
  • colsample_bynode: Subsample columns for each split.
  • num_parallel_tree: Number of trees to train in parallel ("boosted random forest")
• Change updater parameter to only pass a value if user specifies one (fix: Incorrect sequence of updaters #7)
• Add impls of From<String> and From<&str> for TreeMethod.
• Removed some muts to suppress compiler warnings.

I'm keen to hear feedback on the changes for the updater parameter; the main design goal for the change was "make the warning message go away", so would not be surprised if my changes aren't the best way of handling things.

Closes #7