New function: Ultrasound plane wave imaging

[bencox]

Description

• Add new functions that does ultrasound plane wave imaging in 2D and 3D.
• Extend to the case in which the ultrasound plane-wave source is at an angle to the detector array.
• We have matlab code (attached) for the zero-angle case but it will need to be written to fit the coding standard.
• Include examples for 2D and 3D.

US_planewave.zip