nbstripout and "id"

[KrisThielemans]

On a fresh VM 3.1.0-rc.2,we cannot update due to conflicts with the notebooks. Investigating shows

sirfuser@vagrant:~/devel/SIRF-Exercises$ git status
On branch master
Your branch is up to date with 'origin/master'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   notebooks/Geometry/a_geometry.ipynb
	modified:   notebooks/Geometry/b_geom_resamp.ipynb
	modified:   notebooks/Geometry/c_fully3dgeom.ipynb
	modified:   notebooks/Geometry/d_manipulation.ipynb
	modified:   notebooks/Geometry/get_nifti_data.ipynb
	modified:   notebooks/MR/e_advanced_recon.ipynb
sirfuser@vagrant:~/devel/SIRF-Exercises/notebooks/Geometry$ git diff
diff --git a/notebooks/Geometry/a_geometry.ipynb b/notebooks/Geometry/a_geometry.ipynb
index f6627ac..6136965 100644
--- a/notebooks/Geometry/a_geometry.ipynb
+++ b/notebooks/Geometry/a_geometry.ipynb
@@ -2,7 +2,7 @@
  "cells": [
   {
    "cell_type": "markdown",
-   "id": "c4623734",
+   "id": "afa82c11",
    "metadata": {},
    "source": [
     "# Geometry in SIRF"
@@ -10,7 +10,7 @@
   },
<snip>

This is very confusing, as neither the files in that commit, nor on the VM, have the "id" lines. (Note that they only appear in markdown cells).

Even more confusingly it gets resolved by doing nbstripout --uninstall;nbstripout --install.

Not sure how this can happen. Possibly @paskino created the VM in a strange way, but that's hard to imaging (as it's all part of the vagrant up).

No clue what we can do about it, so will probably have to just with the final VM, and potentially add it to the FAQ