Add a visitation model.
Implementing a visitation calculator.

Simple algorithm thats walks the graph of narrower concepts and
calculates the nested sets.

Because we support polyhierarchy, the concept id is nog guaranteed to be
unique. In a nested set that represent a monohierarchy, the formula
max(rght) / 2 represents the number of concepts in that tree. In a
polyhierarchy this would represent the numer of concept positions in
that tree (because a concept can appear in different places in that
tree).
Added conceptscheme_id to thing primary key.
Added a concept_id.

A concept now has a id and a concept_id. ID is the primary key in the
database for the concept table. concept_id is the id of a concept within
a certain conceptscheme.

This means that we can have two concepts with the concept_id 5, but only
one can have the id 5 (and it needn't be one of the two with the
concept_id 5).
Adapted importerer and visit calculator to work with concept_id.
Added a script to calc the visitation of a conceptscheme.
Fix failing unit tests.
Finish visitation calculator.
Fix concept ids in provider.
Added visitation option to provider.

Provider can now use a recursive strategy or a visitation strategy to
deteremine expansion of concepts.
Flake8 fixes.
More flake8 fixes.