SconeExperiment for signature matrix should accept character vectors for poscon and negcon

[drisso]

Something like this should work, but doesn't because it requires poscon, negcon_ruv and neocon_eval to be logical vectors.

poscon <- c("gene1", "gene2", "gene3")

scone_obj <- SconeExperiment(filtered, qc=qc, batch=batch,
                             poscon = poscon)