Create simple 3D velocity model from GemPy Model

[AlexanderJuestel]

Create a simple 3D velocity model from a calculated GemPy Model. The velocities would be assumed independent of depth. That way, an additional column could be added and the respective solution values in the lith_block could be replaced with the respective velocity.

[AlexanderJuestel]

Tutorial 51 illustrates how to assign properties to the lith block of a GemPy Model