Python-weka-wrapper3

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- Added property for attribute indices to `DistanceFunction` class (module `weka.core.distances`)
(thanks to Martin Trat, https://github.com/fracpete/python-weka-wrapper3/pull/39)
- improved instantiation of classes, avoiding misleading output of exceptions
- `JavaArray` class (module: `weka.core.classes`) now has `__str__` and `__repr__` methods that output
classname and size

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- upgraded bundled Weka to 3.9.6

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- switched to `python-javabridge`, the new name (fork?) of the `javabridge` library
- `Package.__str__` (`weka.core.packages` module) method now returns a string rather than printing the name/version
- added `to_numpy(...)` methods to `Instance` and `Instances` classes (module `weka.core.dataset`)
to make it easy to obtain a numpy array from the Weka dataset

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- added method `help_for` to `weka.core.classes` module to generate a help screen for an `weka.core.OptionHandler`
class using just the classname.
- the `to_help` method of the `weka.core.classes.OptionHandler` class now allows to tweak the generated output a
bit better (e.g., what sections to output).
- setting window title of Matplotlib is now dependent on version (to avoid deprecation notice being output)
- `plot_classifier_errors` (module `weka.plot.classifiers`) now plots the diagonal after adding all the plot data to
get the right limits

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- added `weka.core.distances` module for distance functions, with `DistanceFunction` base class
- added `avg_silhouette_coefficient` method to `weka.clusterers` to calculate the average silhouette coefficient

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- the `Package` class of the `weka.core.packages` module now has a `version` property to quickly access the version
which is stored in the meta-data; the `metadata` property now returns a proper Python dictionary
- added convenience methods to the `weka.core.packages` module: `install_packages` to install more than one package,
`install_missing_package` and `install_missing_packages` to install one or more packages if missing
(can automatically stop the JVM and exit the process), `uninstall_packages` to remove more than one package in
one operation