Major revision

[pdimens]

• adds more meaningful threading
• uses wgsim as a subprocess to control stdout/stderr channels better

[IMPORTANT] completely rewrote logic to have barcode-first approach to simulation

Previously, the simulator created molecules, droplets, and overall just tried to digitally mimic 10X chemistry. The new approach now first chooses a barcode, then decides how many molecules will be associated with it, then decides from what chromosomes/intervals those molecules will be from. This approach solves a major flaw of the previous implementation: unrelated molecules sharing a barcode can now come from any chromosome/interval in the haplotype, whereas previously these barcode-clashing events only occurred on the same chromosome/interval. This new approach is a more accurate representation of how clashing occurs.