pull-submodules.py general improvement. Added option --onlylibs

[jgalan]

The following updates have been added to pull-submodules.py

• A new option --onlylibs will force the script to pull only the submodules found at libraries directory.
• The repositories that contain data, external or userguide will be now excluded by default.
• A --help or -h option has been added, and documentation has been improved.

For example, we could use it as follows to simplify the pulling procedure, pulling only geant4lib,rawlib, and connectorslib

jgalan@sultan:~/rest-framework$ python3 pull-submodules.py --onlylibs --exclude:axion,detector,track
Framework branch name: jgalan_pull_submodules
source/libraries/geant4[ OK ] (9397cd2)
source/libraries/raw[ OK ] (b15d1cc)
source/libraries/connectors[ OK ] (de207df)
jgalan@sultan:~/rest-framework$ 

Help output has also been reviewed:

jgalan@sultan:~/rest-framework$ python3 pull-submodules.py --help
 pull-submodules.py is a helper script to pull REST-for-Physics related submodules

Usage:
python3 pull-submodules.py --clean
python3 pull-submodules.py --latest
 
 --clean : It will restore all submodules to the official release versions
 --latest : It will pull the master branch of each submodule

When using --clean make sure the framework local repository is at an official release

 Other complementary options:
  --force : It will override changes using git reset
  --dontask : It wont ask when overriding changes
  --lfna : It will pull LFNA Git repositories. SSH grant required
  --sjtu : It will pull SJTU Git repositories. SSH grant required
  --exclude:lib1,lib2 will prevent lib1,lib2 from being pulled
  --onlylibs: It will pull only the REST library submodules