options function for cellpypes

[FelixTheStudent]

Perhaps it makes sense to set options at beginning of pypes like this:

obj %>%
  options(...) %>%
  rule(...) %>% rule(...)

Parameters I can imagine:

• threshold_units: "fractions", "permille", "per10k", "ppm". This way, I could write .1 instead of .1e-3 in all rules, increasing readibility. Also, the feature plot function could display permille/per10k/... in its color legend.
• perhaps pull in meta.data here, so that you can write rules for totalUMI, percent.mito etc.?
• method: "count_pooling", "saver", or others?

[FelixTheStudent]

I decided against the options option to keep cellpypes a simple, agile tool. Selected and well defined functionality.