utils/sim.py: Remove singletons peak from distribution, handle PCR tree depth inequalities, + more. - Change family size distribution to approximate a pre-error one.   - Huge singletons peak normally observed is due mostly to errors. - Specify maximum inequality in PCR tree branch depths.   - Determined by check_tree_balance(), enforced by get_good_pcr_tree(). - Put simulation parameters into separate argparse group. - Let user preserve wgsim output in --frag-file. - Pass script seed to wgsim.
utils/sim.py: Remove singletons peak from distribution, handle PCR tree depth inequalities, + more.
- Change family size distribution to approximate a pre-error one.
  - Huge singletons peak normally observed is due mostly to errors.
- Specify maximum inequality in PCR tree branch depths.
  - Determined by check_tree_balance(), enforced by get_good_pcr_tree().
- Put simulation parameters into separate argparse group.
- Let user preserve wgsim output in --frag-file.
- Pass script seed to wgsim.

makrutenko/dunovo,makrutenko/dunovo,makrutenko/dunovo