Add TURBINE/REPI with EPI-based tools

[Daval-G]

This PR adds the following features:

• initialize_3D_TURBINE function to create a TURBINE trajectory requested in issues #50, that consists in EPI shots rotated into a cylinder,
• initialize_3D_REPI function to create a REPI trajectory requested in issues #49, that can be either radial or spiral EPI-like shots interlaced between blades rather than stacked independently
• epify function used to combine shots into a single multi-echo readout train, similar to EPI shots
• prewind and rewind to create the positioning & spoiling gradients before and after the readouts, and therefore obtain the full gradient sequence rather than just the one played during the readouts.

The different functions & features are illustrated below, with the TURBINE & REPI trajectories (up) and the epify/prewind/rewind tools applied to different trajectories (down)