Solver now takes data explicitly as argument

Prepare the decentralized and parallel version :
one can choose what data one wants to pass to the solver.
One can specify the solver in use at the loop level
Add a shel script to get animated gifs from dumped files
Set period of empty space mission  to reasonnable value
Update constant -- add a param for using weighted map
Fix glpk solver
Add a decentralized version to solve the problem

In this case, robot are planning one after another.
Improve display

- visibility is now displayed only in VERBOSE mode
- dumped file have better names.
Improve utility metrics
Git ignore output files (svg,png,gif)
UtilityMap can be deepcopied
Fix virtual utility_map in mission
Fix period count in mission.py
Increase robot sensor range to 30m
Fix weighted map in AccessibilityMap
Fix display for robot weighted map