differential expression improvements (#452)

* add cutoff for low expression genes in topN selection

* remove debugging printfs

* change cli param name for diffexp cutoff

* change CLI param name

* second try at diffexp - using lfc sort with pval cutoff

* use lfc cutoff

* update comments to match code; cap p-value adjustment to max of 1

* lint

* explain diffexp in readme

* add link

* add diffexp-lfc-cutoff to test config

* update test to match revised diffexp spec

* fix latent bug in GET arg handling that was breaking tests

* lint

* comment cleanup

* fix variance overestimation so it is symmetric

* lint

server/app/driver/driver.py

@@ -17,6 +17,7 @@ class CXGDriver(metaclass=ABCMeta):
         self.layout_method = args["layout"]
         self.diffexp_method = args["diffexp"]
         self.max_category_items = args["max_category_items"]
+        self.diffexp_lfc_cutoff = args["diffexp_lfc_cutoff"]
         self.cluster = None
 
     @property