Iterators

Each class is strongly suggest to implement at least one iterator to explore the data, also consider to implement iterator to explore subset of your data. For example grid implement several iterator to explore the full data or sub-grid, Cell list implements iterator on nearest neighbourhood cell to explore near particle (under test). Two kind of iterator exist

Unstructured It iterate through all the elements, but there is not information about the indexing, basically it work at memory level ignoring the memory mapping of the structure
Structured It iterate over all the elements with the structure indexing

For example a 2D grid has 2 index i,j, but on memory i,j must be transformed (in some way), in 1-D (mapping) k. Unstructured iterate through the valid index of k, Structured iterate through i,j