Merge maps data layers

## Scenario summary
Currently, two layers with city level info are merged at runtime, creating some "holes" with no postal code info

## Problem statement

## Expected behaviour
All reverse geocoding requests in Italy should produce a valid postal code

## Steps to reproduce
Reverse geocode with point: 37.0502,14.87136 does not return a postal code

## Proposed solution
Join Layer8 with postal-codes layer