add feature: ikle2 / ikle3 as variable

[tomsail]

This feature would be handy for selecting and cropping windows of the selafin file.

See an example for crop function in Thalassa

Tasks:

• add the 2D elements as a variable
• add the 3D elements as a variable when dealing with a 3D Selafin
• add the number of elements as a dimension

[tomsail]

suggestion from pmav99:

• maybe not suppress names instances ikle2/ikle3
• add more checks and cases for if not is_2d: # if 3D