move all concerning intervals in proper classes

    Interval2D represents a 2D interval, same as Interval3D
    both can be randomly generated
    Intervals is kind of a placeholder for 2-3D intervals
remove numpy, step can now only be an interger
fix: save and load the interval3D in placeholder

    make this interval3D as field of Intervals
change how 2D intervals are serialized for humans

    add __str__ in interval2D
    change write_2d_intervals() to use it
remove deprecated data files
remove step_size default value
fix: replaces '1's that were in fact step_size
refactor step_size to step
add new interval3D as binary test file