add a plot_network method using NetworkX as backende

[LouieMH]

Added function for plotting network structure of network class using the NetworkX package - allowing for easy integration with Matplotlib graphics and similar.
Visual cusomizability remains limited compared to current Graphviz plots.

[LegrandNico]

Close #127

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.09615% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyhgf/plots/graphviz/plot_nodes.py	83.82%	1 Missing and 10 partials ⚠️
pyhgf/plots/graphviz/plot_network.py	83.78%	4 Missing and 2 partials ⚠️
pyhgf/plots/networkx/plot_network.py	85.71%	3 Missing and 3 partials ⚠️
pyhgf/plots/graphviz/plot_trajectories.py	86.20%	1 Missing and 3 partials ⚠️
pyhgf/model/network.py	80.00%	1 Missing and 1 partial ⚠️
pyhgf/plots/graphviz/plot_correlations.py	86.66%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
pyhgf/plots/graphviz/__init__.py	100.00% <100.00%> (ø)
pyhgf/plots/networkx/__init__.py	100.00% <100.00%> (ø)
pyhgf/updates/prediction_error/exponential.py	100.00% <ø> (ø)
pyhgf/model/network.py	85.29% <80.00%> (-1.07%)	⬇️
pyhgf/plots/graphviz/plot_correlations.py	86.66% <86.66%> (ø)
pyhgf/plots/graphviz/plot_trajectories.py	86.20% <86.20%> (ø)
pyhgf/plots/graphviz/plot_network.py	83.78% <83.78%> (ø)
pyhgf/plots/networkx/plot_network.py	85.71% <85.71%> (ø)
pyhgf/plots/graphviz/plot_nodes.py	83.82% <83.82%> (ø)

[LegrandNico]

Thank you for your work @LouieMH 😊