New functionality: implement geneset enrichment test

[pascaltimshel]

Implement 'geneset enrichment test'
Statistical test: wilcox test on ESmu. Test if genes in gene list has higher ESmu than genes not in the lidt.
Input: gene list(s)
Output: enrichment statistics (stat, pval, conf int, ...) for each cell-type and gene list.
See BMIbrain manuscript for method description.
(See Jon's code for inspiration: https://github.com/perslab/19-BMI-brain-wgcna)