update to version 6

be a bit selfish

convert boolean command line options to stdbool too

switch to long double to increase integer precision

tests: disable code sensitive to math library accuracy

add options to control floating point precision

rewrite command line parsing with getopts

fix command line processing

tests: fallback to $at_dir

tests: log shell state for debugging

CI: migrate from Travis to GH Actions

disable use of yywrap

distcheck: include install testing

README: convert to markdown for GH rendering

store: constify the file/name args