Related to #5 and #12

Merge branch 'develop' of https://gitlab.mbb.univ-montp2.fr/edna/snakemake_rapidrun_obitools into develop

Force rule goodlength_samples execution over all rules
Related to #5 and #12

- remove python code from Snakefile --> move to script/write_demultiplex_files.py and common.smk
- create helper functions in common.smk
- remove flag rules
- add temp(output) in some rules (output delete when used) (related to #5 and #12)