node names restrictions

If (according to 061ec2da) it's no more possible to name a session with a number,
it must be check everywhere:

(bliss) pcsht:~ % bliss -c 3ttr
Creating '3ttr' BLISS session
Creating sessions/3ttr.yml
Creating sessions/3ttr_setup.py
Creating sessions/scripts/3ttr.py


(bliss) pcsht:~ % bliss -d 3ttr

Available BLISS sessions:
-------------------------
Traceback (most recent call last):
  File "/home/guilloud/PROJECTS/bliss/bliss/shell/cli/main.py", line 164, in main
    arguments = docopt(__doc__)
  File "/opt/bliss/conda/miniconda/envs/bliss/lib/python3.7/site-packages/docopt.py", line 579, in docopt
    raise DocoptExit()
docopt.DocoptExit: Usage: bliss [-l | --log-level=<log_level>] [-s <name> | --session=<name>] [--no-tmux] [--debug]
       bliss [-v | --version]
       bliss [-c <name> | --create=<name>]
       bliss [-D <name> | --delete=<name>]
       bliss [-h | --help]
       bliss [-S | --show-sessions]
       bliss --show-sessions-only

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/bliss/conda/miniconda/envs/bliss/bin/bliss", line 11, in <module>
    load_entry_point('bliss', 'console_scripts', 'bliss')()
  File "/home/guilloud/PROJECTS/bliss/bliss/shell/cli/main.py", line 169, in main
    print_sessions_list(get_sessions_list())
  File "/home/guilloud/PROJECTS/bliss/bliss/config/__init__.py", line 28, in get_sessions_list
    config = static.get_config()
  File "/home/guilloud/PROJECTS/bliss/bliss/config/static.py", line 211, in get_config
    CONFIG = Config(base_path, timeout)
  File "/home/guilloud/PROJECTS/bliss/bliss/config/static.py", line 519, in __init__
    self.reload(timeout=timeout)
  File "/home/guilloud/PROJECTS/bliss/bliss/config/static.py", line 627, in reload
    self._create_index(parents)
  File "/home/guilloud/PROJECTS/bliss/bliss/config/static.py", line 844, in _create_index
    f"Invalid name ({name}) in config file ({node.filename}). Must start with [a-zA-Z_]"
ValueError: Invalid name (3ttr) in config file (sessions/3ttr.yml). Must start with [a-zA-Z_]
(bliss) pcsht:~ %