Support for slicing signals

[gfngoncalves]

This moves the logic of the train test split to the signal classes, which should clean up the code a bit, while also adding support for indexing signals with slices.

[codecov-commenter]

Codecov Report

Merging #160 (d673730) into master (2ffd5fe) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
- Coverage   99.31%   99.31%   -0.01%     
==========================================
  Files          57       57              
  Lines        4994     4984      -10     
==========================================
- Hits         4960     4950      -10     
  Misses         34       34              

Impacted Files	Coverage Δ
...ric_temporal/signal/dynamic_graph_static_signal.py	97.43% <100.00%> (+0.06%)	⬆️
...mporal/signal/dynamic_graph_static_signal_batch.py	95.23% <100.00%> (+0.11%)	⬆️
...c_temporal/signal/dynamic_graph_temporal_signal.py	100.00% <100.00%> (ø)
...oral/signal/dynamic_graph_temporal_signal_batch.py	95.29% <100.00%> (+0.11%)	⬆️
...poral/signal/dynamic_hetero_graph_static_signal.py	98.90% <100.00%> (+0.02%)	⬆️
...signal/dynamic_hetero_graph_static_signal_batch.py	100.00% <100.00%> (ø)
...ral/signal/dynamic_hetero_graph_temporal_signal.py	100.00% <100.00%> (ø)
...gnal/dynamic_hetero_graph_temporal_signal_batch.py	100.00% <100.00%> (ø)
...ic_temporal/signal/static_graph_temporal_signal.py	100.00% <100.00%> (ø)
...poral/signal/static_graph_temporal_signal_batch.py	95.12% <100.00%> (+0.12%)	⬆️
... and 3 more

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[benedekrozemberczki]

Thank you @gfngoncalves!