IDs of database tables should be contiguous and start at 0

[pdigiglio]

The underlying data structure of the *Table classes is now a std::vector (instead of a std::map). This means that the IDs of the input database tables must be contiguous and should start at 0.

If the IDs don't start at 0, an artificial shift can be introduced between the database ID and the vector index, e.g. through the function idToVectorIndex:

class DBTable {
public:
    // ...
private:
    /// The vector of database rows.
    std::vector<T> Vector_;

    /// Helper function to convert database ID's to vector indices.
    virtual const size_type idToVectorIndex(const index_type& id) const;
};

So far the ID shift is hard-coded, so it would be much easier if all the table IDs started at 0.

[pdigiglio]

Actually it's better if the table IDs start at 1. This way we can use the ID 0 as an invalid ID.