Added Morsy, a new lightweigth robot model for testing

The model may be used for education material or MORSE demos for
instance.

Based on the official MORSE mascott ;-)
Thanks to Michael Karg for the initial Blender version
[builder] Minor improvement to error msg when a model is not found
[bin] Separate set_prefixes from check_setup

This allows to detect and set the various MORSE prefixes without actually launching
Blender for the sanity check.
'morse {init|rm|add}' to help with simulation creation

This commit proposes new command-line tools to create initial
'simulation environments' that
aim at simplifying and formalizing the code/resource layout
of a MORSE simulation.

The commit also introduces the idea of 'simulation environments' that
are directories containing all required resources for a simulation
project.

The tool currently implement 3 modes: init, rm and add

* When running 'morse init my_simu', the tool creates a new
directory called 'my_simu' with a template MORSE scene (my_simu.py).

If a environment with the same name was previously defined, the user
must use 'morse init -f ...' to force overwriting.

The scene can directly be run with 'morse run my_simu' (or optionally
with 'morse run my_simu other_file.py') (current usage of 'morse run
<file>' is preserved as well).

A test client (scripts/my_simu_client.py) that uses pymorse is also
generated.

* 'morse rm [-f] <env>' deletes an existing environment

* 'morse add [robot|sensor|actuator] name [env]' inserts templates for
new components. WIP! CURRENTLY ONLY ROBOT ADDITION WORKS.

You can get details by running 'morse <mode> --help'.

The commit also propose an initial 'simulation environment' template
located in $MORSE/data/templates. Improvements are welcome.
Added man pages for each of the morse subcommands