Functions to read the overarch input models from filesystem.

Functions to read and build the working model from overarch input models.
   
The built model is a map with the following keys

| Key                     | Description 
|-------------------------|-------------
| **Relation keys**       | 
| :nodes                  | the set of all nodes
| :relations              | the set of all relations
| :views                  | the set of views
| :themes                 | the set of themes
| **Index keys**          | 
| :id->element            | a map from id to element (nodes, relations and views)
| :id->parent-id          | a map from id to parent node id
| :id->children           | a map from id to a vector of contained nodes (deprecated)
| :referrer-id->relations | a map from id to set of relations where the id is the referrer (:from)
| :referred-id->relations | a map from id to set of relations where the id is referred (:to)
| **Problem keys**        | 
| :problems               | the set of problems found during model building

The input model is transformed by
* computing missing ids, if possible
* converting the node hierarchies to :contained-in relations
   

Functions for reading and building models and setting the repository state.
The multimethods should be implemented by specific readers.