Central loop with square source loop

[prisae]

Every now and then there comes a question regarding central loop for a square source loop.

This is possible with empymod, as shown here:
https://empymod.emsig.xyz/en/stable/gallery/tdomain/tem_walktem.html

However, it would be good to implement this better/easier for the user in empymod.

One can use loop for it, but that assumes a circular loop. So maybe simply a flag whether the loop is circular or square would be sufficient.