Inappropriate hard-coded constant const.MAX_TOTAL_DROPLETS_GUESSED for large datasets?

[colobas]

While running cellbender on a large dataset, I kept running into problems similar to what's described in #343 and #190 .
I boiled it down to the fact that this condition was never met because my dataset is very big. I realized in practice max_total_droplets is hardcoded to 70000 (in the get_priors function signature, max_total_droplets has a default of const.MAX_TOTAL_DROPLETS_GUESSED, which is set to 70000 here)

Another user described having issues with a big dataset too, which I would guess is a similar situation to mine.

I'll open a PR shortly to try and address this