Add examples and scripts for 3D simulations

[sigurdstorve]

Although the simulator is fully capable of 3D simulations, all examples are currently 2D-only, and there
is only a single Matlab script for using Field II to simulate beam profile lookup tables, which is a 1D
phased array probe. The analytic Gaussian beam profile can be used for 3D simulations - all that is
required is to add an example for this.

Outline of steps required to support 3D scans out-of-the-box

• Add a new Python example that uses an analytic Gaussian profile to simulate a 3D scan of, e.g.,
  the left ventricle phantom. Then some simple visualization must be added, e.g. some orthogonal
  plane slices?
• Create a Matlab script for using Field II to simulate a 2D array matrix. Hopefully much of the
  existing Matlab code is reusable, as the code already calculates the sensitivity in a 3D grid.
  The xdc_2d_array function should be a good starting point.
• Extend the 3D Python example to also use the simulated 2D array lookup-table.
• Perhaps extend the GUI to support visualizing 3D datasets (this is probably a fair amount of work..)
• Perhaps Add support for multiple beam lookup-tables #64 ties into this as well? The realism of a 3D simulation would probably be increased if beam-steering effects are also included.