PerformanceMonitoring: First batch of cmake, flake8, clean-up updates

This relates to ATEAM-678. The original goal was to enable flake8 for the packages under Control/PerformanceMonitoring. While doing so, I tried to clean things up that I'm fairly certain not used and/or needed anymore, although I played it safe. I also went through the cmake configurations, fixed a number of py3 problems in some scripts that are not used very frequently etc. It turned out to be a bit more massive MR than I was intending but pretty much all changes are either stylistic and/or uncontroversial. There is essentially no new functionality.

BTW, there are a few commits but it would be nice if we don't squash them just in case.

Control/PerformanceMonitoring/PerfMonAna/CMakeLists.txt

@@ -6,8 +6,11 @@ atlas_subdir( PerfMonAna )
 # External dependencies:
 find_package( pandas )
 find_package( sqlalchemy )
-find_package( matplotlib )
-find_package( numpy )
+# These need to be added to Externals first, then enabled
+# However, the fact that they're missing is not an immediate
+# Showstopper...
+#find_package( matplotlib )
+#find_package( numpy )
 find_package( ROOT COMPONENTS Core PyROOT Tree MathCore Hist RIO pthread )
 
 # Install files from the package: