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)