Added :class:.DiGraph, a mobject representing directed graphs (#2974)

* Directed graphs

* use type instead of isinstance

* respect object boundary

* refactor graph with GenericGraph

* update tests

* fix doc string

* custom tips override default, and more documentation

* improve docstring situation, move examples to concrete classes

* further improve docstrings

* refactor: remove graph_type, simplify handling of tip_config

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* removed superfluous import

* fixed two tests

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Co-authored-by: Benjamin Hackl <devel@benjamin-hackl.at>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people