Issue 209: Add layers accessors

[ftomassetti]

See issue #209

Note that the accessors return the whole layer, then the data field has to be accessed explicitely. Also theresholds and quantiles can be accessed from the layer.

[tcld]

This looks so much nicer and cleaner than before! I have not checked every line yet.

One thing I mentioned before: Why not store the thresholds in loops? (I'll add an example comment for that.)

EDIT: I have looked at all of it now; looks good. But the setters should be part of this PR, I think.

[tcld]

I would much prefer this when it comes to storing thresholds:

for pair in world.permeability.thresholds:
    permeability_ths_grp[pair[0]] = pair[1]

As far as I understand the item at [x][0] is a naming string anyway. So far I haven't seen those strings used anywhere - so they could be renamed to 'low', 'med' etc., if necessary. Then the stored file would be the same as before, but the whole system could be expanded/changed more easily.