Merge branch 'feat-som-speedup' into develop

src/apollon/aplot.py

src/apollon/hmm/poisson_hmm.py

src/apollon/io/io.py

src/apollon/som/datasets.py

src/apollon/som/neighbors.py

src/apollon/som/plot.py

src/apollon/som/som.py

src/apollon/som/topologies.py

src/apollon/som/utilities.py

src/apollon/tools.py

src/apollon/types.py

tests/som/test_neighbours.py

tests/som/test_som.py

tests/som/test_som_utilities.py