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/scanpy_engine/scanpy_engine.py

@@ -325,8 +325,8 @@ class ScanpyEngine(CXGDriver):
             raise FilterError(f"Error parsing filter: {e}") from e
         if top_n is None:
             top_n = DEFAULT_TOP_N
-        result = diffexp_ttest(self.data, obs_mask_A, obs_mask_B, top_n)
-        return sorted(result, key=lambda r: r[0])
+        result = diffexp_ttest(self.data, obs_mask_A, obs_mask_B, top_n, self.diffexp_lfc_cutoff)
+        return result
 
     def layout(self, filter, interactive_limit=None):
         """