pseudobulking of counts by metadata feature

[sreichl]

• [x] new config field(s) to generate a simple pseudo count matrix for downstream analysis using bulk methods (breaking change)
  • probably a list of categorical metadata
  • pseudobulk: by: ['patient', 'cellType', 'treatment'] method: "sum"
• [x] look how others do it
• [x] support multiple methods
  • sum
  • mean
  • median
• [x] support modalities
  • [x] RNA
  • [x] AB
  • [x] grna
  • [x] custom
• [x] generate metadata sheet
  • include statistics of pseudobulked cell numbers per sample
• [x] document r-dplyr=1.1.2