Big refactor: Use affine matrice to locate arrays

[paquiteau]

This PR changes how Phantom (e.g spatial arrays) are considered in SNAKE.

• First the simulation config now has a fov field, that contains all parameters to locate an array in the RAS referential.
• Phantom (initialized from BrainWeb) also have a affine matrix field (BrainWeb data is in fact in MNI152 space)
• The FOV is applied in the Engine acquisition, and the "Samplers" generating trajectories are aware of it.
• It also fixes the contrast in the 2D mode (it is now computed with a TR_eff, set to the volumic TR in case of 2D excitations)

Some further improvement

• Add examples about motion (using the new FOV system, it is in fact fairly easy to do !)
• Add unit test setup for file writing and loading