Refactor modifiers

Hi,

Here are some commits that:
- refactor modifiers into a class-based structure
- clean the noise modifiers
- update the builder data dictionnaries
- generate doc for modifiers
- unit tests for modifiers
- add a 'make_ghost' method to the builder to make a robot transparent and with no collision
- add some doc for modifiers + a tutorial for noise modifier and make_ghost usage

Fix issues #330 and #287.

The remaining issue:
- the odometry noise is not up to date, because I had some troubles with it: it should try to use the levels of the Odometry sensor...

Warning: This pull request contains much more commits than it should: I tried to make some rebase on master, but it then contained less but different commits... the commits to pick are mines from March 26th.

I also would like to make a small video of the noise-and-ghost tutorial: any tips on how to proceed? (directly from Blender, using external recorders, ...)

Comments welcome