Fixed the get_data_size() methods for multi-point features to check properly for end 
of their respective data arrays.
Extended the point checking function to take a 3d optional argument so cases where
there's additional data in each array element (besides the point data itself) can be
covered by the helper function.
Fixed the 3 cases where such offset was present to use the proper checking helper 
function.
Test cases added.
Fixed review comments.