Enhanced source functions

[prisae]

The current source functions use either:

• The adjoint of tri-linear interpolation for points;
• Allocating arbitrary shaped sources to the relevant cells.

Better source functions would be great. E.g., the adjoint of cubic interpolation for points would allow to use cubic interpolation at receivers and still be able to create the adjoint-state gradient.

[prisae]

• When cubic interpolated source is implemented, the default in Simulation should change.
• Have a look at the Rbf-functionality within SciPy - can we get out the weights to do the adjoint?

[prisae]

The best would be to implement cubic interpolation into discretize, and then use discretize and its adjoint to construct the source function.