fix: underrun bug: add: %_ expanding space, predesigned progress bars
more testing of test code (not included in PyTest)
split issue4 data from test logic
add: docs, custom_expanding_outputter
add: issue4_result to test data
IntervalTree inherits from collection ABCs
upd: changelog [ci skip]
add to test.intervals: pbar, rename vars
refactor
moved imports, load from test.intervaltrees
add to optimality_test_matrix: progress_bar