Clear out MyPy's warnings

[e10e3]

When type checking the code, MyPy raises some warnings:

river/drift/kswin.py:112: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
river/tree/nodes/sgt_nodes.py:45: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
river/tree/nodes/hatc_nodes.py:140: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
river/tree/stochastic_gradient_tree.py:56: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
river/forest/aggregated_mondrian_forest.py:173: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
river/forest/aggregated_mondrian_forest.py:293: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
Success: no issues found in 465 source files

These warnings are the result of inconsistencies in the annotations, but are not errors because MyPy does not check untyped functions by default.

The inconsistencies can be cleared by adding or correcting annotations: with the changes of this PR applied, no warnings are raised any more by MyPy.

[smastelini]

Thanks for looking into that, @e10e3 ! LGTM!