Adding foreach to pull-submodules.py

[juanangp]

Added git submodule foreach to pull-submodules.py script:

• Avoids multiple request to github that might cause issues in unizar firewall
• Speed up the checkout of the submodules

Note that the overall behaviour of pull-submodules.py has not been significatively changed, however note that git submodule foreach acts on all the submodules that have been initialized.

[jgalan]

I did a quick test of the pulling process, but it seems the output has been strongly affected

[jgalan]

This is how it looks in master

[juanangp]

This is how it looks in master

Indeed, the output now is tricky because git submodule foreach gets the info for all the submodules at the same time.

I have changed it, now it gets the info at the different steps and it prints the hash after it is done, see below:

[jgalan]

This PR needs approval