REVERTME: debug code
reduce calls to PLTSItem.is_taken_or_dead()
PLTSBlock may create dead blocks
fix: ensure end index is never < 0
REF: rename method first->start
where possible, obtain iterators more efficiently
do not allow directories as benchmark input

Having the benchmark output of several graph instances in one file would be very confusing and a source of mistakes
allow only one file as benchmark input

same reason as with directories
script for benchmarking: parse arguments
FMT
rename option 'seq' to 'sequential'

to be consistent with the other options
benchmarking script: run benchmarks
remove debug code
call make before executing benchmark
do not print benchmark parameters in results
remove some old scripts
REF
some convenience options for the benchmarks script