Relation filtering for edge methods + sparse metapath counting (#12)

* filter relations for edge card; cap mp workers based on cores

* add relation filter to edge methods

* ci fix

* metapath tweaks

* refactor metapath counting with sparse matmuls

* docstring update

* use np.divmod

* avoid repeated work

* add metapath unit test

* reduce memory usage of metapath counting

* improve docstrings

* document new functionalities in the doc notebook

* tidy up redundant code

* fix typo

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Co-authored-by: Daniel Justus <[email protected]>