scDRS breaks if using all scRNA-seq genes as disease genes

[martinjzhang]

Some users may use all scRNA-seq genes as the trait gene set as some sort of negative control. scDRS produced miscalibrated results in this case due to numerical issues.

• Raw control scores were slightly but systematically smaller than the disease score (1e-17).
• Small SD of raw control of a given cell across control gene sets (1e-18).