Use "algorithm" instead of "model" and dispatch predict on kind of target proxy

[ablaom]

Closes #12 and #14.

In this PR:

• Rename the structs that store the hyperparameters of some ML/statistics algorithm, algorithms (previously models)
• Remove predict_joint method, as redundant under changes below
• TargetProxy abstract type is renamed KindOfProxy
• Make predict for target variable or target proxy only (transform is for anything else) and add argument to predict to allow dispatch on the kind of target proxy. In this way algorithms can implement predict for multiple kinds of target proxy, although we now require declaration of a preferred kind (new preferred_kind_of_proxy trait).
• Remove traits for input/output type/scitype of inverse_transform; I figure these can just be inferred from the transform traits.
• Update all associated documentation