[doc] add an example for modifying flat field from scratch

doc/tutorials/patch_nx.rst

@@ -83,3 +83,49 @@ Those are some examples of usage:
           * modify frame to `dark` type if any
           * modify frame to `invalid` if any
 
+
+Concrete example of modifying flat field on an .nx file
+-------------------------------------------------------
+
+In this example flat field frames at from acquisition A has been mess up.
+NXTomo file is name original_acquiA.nx. Entry name is entry000A
+
+And we want to replace them by flat field from acquisition B (master file is acquiB.nx). Entry name is entry000B
+
+This is one way to proceed to replace flat field frames:
+
+1. go to acquisition A folder containing the acquiA.nx file
+
+    .. code-block:: bash
+
+        cd [path_to_acquisitionA_folder]/acquisitionA
+
+2. copy the original .nx file. Modification are in place. This is safer to copy the file.
+
+    .. code-block:: bash
+
+        cp original_acquiA.nx acquiA.nx
+
+3. invalidate the flat field frames made at start. In the case let say that we want to invalid frames from 20 to 40 included:
+
+    .. code-block:: bash
+
+        nxtomomill patch-nx set83_tomo_black_drum_LPJ01_6p5p_20N_0001_0000_patch.nx entry0000 --invalid-frames 20:41
+
+4. check that the invalidation of frames worked properly using silx view for example
+
+5. get the silx url you want to link as the new flat field. Syntax is `silx://[file_path]?path=[data_path]&slice=[slices]`
+   for example here we want to link frames 2000 to 2021 from acquiB.nx
+   So the url looks like: silx://[folder_to_acquiB]/acquiB.nx?path=/entry000B/instrument/detector/data&slices=2000:2021
+
+6. then patch flat from patch-nx command and the 'flats-at-start' option:
+
+    .. code-block:: bash
+
+        nxtomomill patch-nx acquiA.nx entry000A --flats-at-start "silx://[folder_to_acquiB]/acquiB.nx?path=/entry000B/instrument/detector/data&slices=2000:2021"
+
+    .. warning::
+        when you provide the url make sure you use `"` or `'` characters. Otherwise in this case the command will be executed as a background task and slices will be ignored.
+        It will also try to link it with the full dataset at `/entry000B/instrument/detector/data`
+
+7. check that your dataset is complete (using silx view for example)