Merge branch 'simulate'

* simulate: (22 commits)
  Test that splat output is a valid SCESet
  Add single mode to splat
  Allow different DE parameters for each group/path
  Add progress messages
  Add seed paramater and seed simulation
  Rename n.genes, n.cells, n.groups for consistency
  Modify outlier expression to use median
  Run checks
  Add simulation documentation
  Add simPathCellMeans function
  Add expandPathParms function
  Add simPathDE function
  Add extra checks checkParams
  Fix bug in mergeParams
  Add simDropout function
  Add simTrueCount function
  Add simBCVMeans function
  Add simGroupCellMeans function
  Add simulateLibSizes function
  Add simulateDE function
  ...