Add logging to FDTD code

[gomezzz]

Feature

Desired Behavior / Functionality

For easier debugging and understanding what happens on the inside we need to add logging to the FDTD part.

For an example how this can and should look have a look here

(If you wanna see what this gives you run on the existing notebooks with loglevel trace or debug ;) )

What Needs to Be Done

• Add logging to fdtd compute_spectrum
• Add logging to init_fdtd
• Add logging to calculate_transmission_reflection_coeffs.py

How Can It Be Tested

In the notebook set different loglevels with nidn.set_log_level and have a look.

[gomezzz]

@torbjornstoro so I think logging for compute spectrum is ok for now the other two don't have any logging yet. Feel free to add in #54 or leave the issue open for now :)