Develop a way to confirm ell-grid-size

[jam31118]

Considering implementing printer for ell-spectrum in a regular basis during the propagation. Fetching the ell spectrum at the end of the post-propagation nor even right after the field became off doesn't really confirm the ell population directly since it is likely that the portion of the state function, especially the fast, high-speed (thus momentum) component has already arrived and went beyond the R-tsurff and thus entered the absorbing potential.