Directory reorganization and sketch of new versioning procedure

doc/03_overlap_sum.md

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+### Aereal Weighting
+
+Aereal weighting is a simple interpolation technique to assign a value
+to a polygon given a set of polygons with one value assigned to each one.
+
+The value is assigned by averaging the values of intersecting areas
+weighted by the intersection area.
+
+Its accuracy depends on the values assigned to reference areas being
+homogeneous over each area.
+
+The `cdb_overlap_function` takes three required parameters:
+
+* `geometry` a Polygon geometry which defines the area where a value will be
+  estimated.
+* `table_name`: name of the values table that provides the source values;
+  this table must have a geometric column `the_geom` containing the polygons
+  to which values are assigned.
+* `column_name`: name of the column that contains the values in the values
+  table (should be a numeric column)
+
+There's also an additional optional parameter to define the schema to which
+the values table belongs. This is necessary only if it is not in the
+`search_path`. Note that `table_name` should never include the schema in it.
+
+* `schema_name` name of the schema that contains the values table
+
+This function returns a numeric value resulting from the aggregation
+of the polygons in