0.5.0 release: refactoring, metric learning, new oversampler

In the 0.5.0 release:

1. A decent refactoring was carried out by splitting the individual oversamplers to separate files.
2. SYMPROD added as the 86th oversampler implemented, thanks to @intouchkun
3. An experimental feature: metric learning to determine the metric to be used for neighborhood estimation added to 69 oversamplers.
4. A sample notebook to illustrate metric learning backed oversampling was added.
5. Support and CI pipelines for Python 3.9 and 3.10 added.