Improve algorithms and simplify code using other functions in xtgeo

[jcrivenaes]

The current code base is not easy to maintain, and uses important (and "complicated") functions that are placed in xtgeo.

Some protoype tests using the newly refactored xtgeo.surface_from_grid3d() function shows both a speedup potential (possibly factor 10, given some adjustments in xtgeo), and (as important) a much simpler code.

See this in context with #127