Update plotting API to plot 2-D sections by position, not cell id

[flohorovicic]

Is your feature request related to a problem? Please describe.
Problem: position of section plot becomes more difficult to determine for ochre refinements.

Describe the solution you'd like
Add option to determine section by value, not cell id, e.g.:

p2d = gpv.plot_2d(geo_model, position=[500], alpha=0.2)

Describe alternatives you've considered
Hack using model extent, initial resolution and octree refinement. Works obviously, but not intuitive (especially not for beginners).

Additional context
n/a