vidjil-algo: prepare CHANGELOG for release 2019.05

see #3914

doc/CHANGELOG

 
 This changelog concerns vijil-algo, the algorithmic part (C++) of the Vidjil platform.
 
+2019-05-XX  The Vidjil Team
+	* Better method to build consensus sequences used by default, old behaviour with --consensus-on-longest-sequences #3866
+	* Optimized analyses with pre-computations, including for e-value estimation !468
+	* Updated functional .should tests with new syntax, renaming and moving some tests #3206 #3762
+
 2019-04-04  The Vidjil Team
 	* New --config and --label-json experimental options (vidjil.cpp, lib/CLI11_json.cpp) #3837 #3839
 	* Improved computation of consensus sequence when there are many reads (vidjil.cpp) !448