Remove module import/exports from top-level __init__ where appropriate

[mferrera]

There are many instances where modules are imported to the top-level init to be re-exported with a shorter import path. An example:

from xtgeo.surface import regular_surface
# Now can access xtgeo.regular_surface

I suspect these can all be removed without breaking anything, but it's possible that this is a breaking change out in the wild. This change should be grouped into xtgeo 4.0.