Initial implementation of 'morse create <env>'

This commit proposes a first implementation of a
tool to create initial 'simulation environments' that
aim at simplifying and formalizing the code/resource layout
of MORSE simulation.

The commit also introduces the idea of 'simulation environments' that
are directories containing all required resources for a simulation
project.

When running 'morse create my_simu', the tool creates a new
directory called 'my_simu' with a template MORSE scene (scene.py),
and and a template for a custom robot.

If a environment with the same name was previously defined, the user
must use 'morse -f create ...' to force overwriting.

The scene can directly be run with 'morse run my_simu' (or optionally
with 'morse run my_simu my_scene.py') (current usage of 'morse run
<file>' is preserved as well).

The commit also propose an initial 'simulation environment' template.
Improvements are welcome.