_Augmenter should be exposed properly as tsaug.Augmenter

[Holt59]

Might be related to https://github.com/arundo/tsaug/issues/1

In the current state of the package, the _Augmenter class is an internal class that should not be used outside of the package itself... but it's also the base class for all usable classes from tsaug. This makes it very weird to type "generic" functions outside of tsaug, e.g.

# this should not appear in a normal Python code
from tsaug._augmenters.base import _Augmenter

def apply_transformation(aug: _Augmenter):
    ...

The _Augmenter class should be exposed as tsaug.Augmenter so that it can be used for proper typing outside of the tsaug package.