The inventory request is supposed to return a distinct list of a list of concepts.
Additionally one may request to count the number of records for each distinct combination of concepts.

---+++ Problems accountered
   * In this request there is no natural "root" table. All concepts are equal and therefore a left join to link the tables is not appropiate. A regular join is implemented. This brings the following problems:
   * It is impossible to retrieve NULLs in the response
   * If I ask for example just for distinct specimen names in the database, there is no relation to a specimen in the query anymore and ALL names will be returned, regardless if there is a specimen belonging to it. To prevend this the query must specify a filter with for example: SPECIMEN_ID IS NOT NULL. This forces the query generator to include the specimen table and to make sure there is a specimen attached to a name.
   * Multiple db attributes mapped to a single concept. It should be possible to map several db attributes or literals to a concept. For example some databases have 2 columns for collector1 and collector2 which both map to the concept "collector".
      * Is it possible to generate a proper SQL in this case? 
      * Are UNIONS needed? 
      * how can the distinct combinations be counted?
      * *!!! Currently an error is raised if an inventory is done on a concept that has several mappings !!!*