readObjectsData.py: Can animate with large files.

Problem: When datafiles got too huge, letting the program read all data
from all files before starting plotting is stupid. Therefore, also
implemented possibility to read one and one point. Instance has a field
(integer) telling how far in the file it has come.

Potential new problem: Reader has to loop further and further every time
its going to get the new point. Animation will go poorly.