Fix cil astra

[paskino]

Fixes updates for CIL ASTRA git-failing because an untracked file is in the sources.

Tested on updating SIRF 3.1.0 VM downloaded from zenodo with the command

cd devel/SyneRBI_VM
git fetch --all
git reset --hard
git checkout update_from_superbuild
# clean the SIRF-Exercises.
# if the machine has not been used it's easier to delete the SIRF-Exercises directory
cd ..
rm -rf SIRF-Exercises
.SyneRBI_VM/scripts/update_VM.sh -s -t origin/fix_cil-astra -j 1

This updates SIRF to 3.3.0 (master).

Still it doesn't update CIL, for instance, but it does rebuild ISMRMRD to 1.7 with support for the utilities, thereby fixing #745

[paskino]

Failures are (around line 10000)

CMake Error: Error processing file: /home/runner/work/SIRF-SuperBuild/SIRF-SuperBuild/build/builds/CIL-ASTRA/tmp/CIL-ASTRA-gitupdate.cmake

[KrisThielemans]

Best to document in the file why this needs to be done. We will forget...

Does cmake -E rm accept multiply files to make this a bit shorter?

[KrisThielemans]

do we really need to call to the git-update? Seems to rely quite heavily on internals of the External project stuff

[paskino]

The rationale is:

1. I need to remove a few files from the source code otherwise git complains
2. I also want to proceed doing the standard git updates stuff that happen in ExternalProject_Add

As far as I understand, there is no command I can run before update. So I am hijacking it to first remove the unwanted files, then continue with the standard update that would be done in ExternalProject_Add

But maybe this will only work for the 3.1.0 VM?