Simplification of the database structure
The database structure of the exposure-lib should be reduced to the minimum for storing exposure data. Currently it provides a mix of pure exposure data and data used for generating these data (e.g. for the simplified Taiwan model). As a consequence, no detailed information about tiles or buildings need to be stored. Also, the District table appears to be obsolete for the purpose of distributing exposure on a tile- or building-level. The extra tables shall not be generated as default (see #16 (closed)) but become available for extended use cases when necessary.