Serde JSON defaults don't match Default::default()

[montanalow]

By default, #[serde(default)] uses the default for the type, e.g. 0 for all numeric types, rather than the default for the structs field, e.g. 3 for KNNRegressor.k.

I will take up a fix to sync the JSON and Default values for all fields, as soon as we agree on a solution for #174. I believe the b option also makes this simpler as well.