Allow for interpolation between source zones

[JoYvBa]

Introduce ability to refine source zone by interpolating between given zones and concentrations. Either linearly, or as a curve.

[JoYvBa]

With introduction of solution from Karanovic et al. (2007), this feature is not needed until the calculation times are significantly shorter. Karanovic solution with a reasonable resolution (+- 130 in x direction, 50 in y and 30 time steps), takes 9 seconds with 3 source zones. Increasing amount of source zones by method of interpolation will significantly increase the runtime to unreasonable amounts for an analytical solution (in my opinion).