S K-edge normalization

This change is to accommodate the case (encountered at the sulfur K-edge) where only the sum of 'n' and 'p' ROI can be normalized against '0'.

Changes

• depreciate inspect_plane_fitting and replace it with inspect_ff_fitting
• framework to work with different flat field normalization function
• implement 2D polyline normalization
• compute np_mean directly in process

doc/changelog.rst

@@ -23,7 +23,7 @@ unreleased
 - **New Features**
 
     - fix :issue:`75` add frame counts when aggregating RIXS data :mr:`274`
-		- BOZ normalization using 2D flat field polylines for S K-edge :mr:`293`
+    - BOZ normalization using 2D flat field polylines for S K-edge :mr:`293`
 
 1.7.0
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