refactor: Split definition and implementation of graph classes

[bobluppes]

• Split implementation and definition in all headers
• Added missing includes
• Fixed some typos in javadoc

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (033b366) 100.00% compared to head (56c092b) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         9    +1     
  Lines          414       411    -3     
=========================================
- Hits           414       411    -3     

Impacted Files	Coverage Δ
src/graaflib/algorithm/graph_traversal.tpp	100.00% <ø> (ø)
test/graaflib/graph_test.cpp	100.00% <ø> (ø)
src/graaflib/directed_graph.tpp	100.00% <100.00%> (ø)
src/graaflib/graph.h	100.00% <100.00%> (ø)
src/graaflib/graph.tpp	100.00% <100.00%> (ø)
src/graaflib/undirected_graph.tpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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