[feature] add subspace projection model.

[mcencini]

This PR add a new extension to enable low rank subspace projected NUFFT as described (in a Cartesian context) here. A non-exhaustive list potential applications is:

• Multi-contrast MRI with virtual echoes (e.g., Tamir et al.)
• Acceleration of iterative reconstruction for quantitative MRI (e.g., Magnetic Resonance Fingerprinting, MR Multitasking)
• Dynamic (e.g., cardiac) MRI