Improved BOZ flat field

• adds low and high threshold on product and ratio of ROIs to calculate flat field correction
• adds default flat field guess for the fit as parameters
• implement hexagonal lattice for DSSC pixels
• breaks left/right symmetry for flat field fitting since zone plate can be slightly rotated
• implements a refine fit with regularization for flat field calculation
• implements dask auto rechunking

Closes #53 (closed) #55 (closed)

doc/changelog.rst

@@ -13,6 +13,8 @@ unreleased
     - fix :issue:`51` make sure that BAM units are in ps :mr:`183`
     - fix :issue:`50` and :issue:`54` relating to package dependencies
 		- fix :issue:`57` adds target mono energy mnemonic
+    - fix :issue:`55` implementingd dask auto rechunking in notebooks
+    - fix :issue:`53` wrong flat field correction sometimes being calculated 
 
 - **Improvements**
 
@@ -21,7 +23,11 @@ unreleased
     - reorganize the Howto section :mr:`169`
     - improve SLURM scripts with named arguments :mr:`176`
     - adds notebook for DSSC fine timing analysis :mr:`184` and :mr:`185`
-  
+    - numerous improvements for the flat field correction calculation
+      in the BOZ analysis, including fitting domain functions, hexagonal
+      pixel lattice, possibility to switch off flat field symmetry constraints
+      and a refine fit function with regularization term :mr:`186`
+ 
 - **New Features**
     - add routine for fluence calibration :mr:`180`