flake8: exmaralda.py
refactoring: outsourced generic functions from exmaralda.py to discoursegraph.py
- get_annotation_layers()
- get_span()
- get_text()
- select_nodes_by_layer()
- select_edges_by_edgetype()
- get_pointing_chains()
added exmaralda output option to merging cli / makefile; refactored imports