Store gene parameters in data.frame rather than separate vectors. Forces them
to be same length and allows more independence between gene parameters and
nGenes. Checking for differences between the number of gene parameters and
nGenes, and sampling if necessary, is now done in `lun2Simulate` rather than
`validObject`. The same is true for nCells and lib.sizes.

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/splatter@125902 bc3139a8-67e5-0310-9ffc-ced21a209358