Agipd corr mod

Summary:

These are changes introduced by @jsztuk for AGIPD correction

• Relative gain correction modification.
• exposing common-mode configurations.
• updates for the within code documentation.

The MR was rebased with the master and I have refactored @jsztuk 's chages and it is ready for review

@danilevc @kamile

cal_tools/cal_tools/agipdlib.py

@@ -413,6 +413,7 @@ class AgipdCorrections:
                 # if not we continue with initial data
                 else:
                     dd = data[i]
+                    sh = 0
 
                 # if we have enough pixels in medium or low gain and
                 # correction via hist matching is requested to this now