Create a comparison example with gprMax, to

• verify correctness of empymod.model.gpr,
• get an idea how they compare in terms of speed.

As comparison could serve these examples:

• https://github.com/empymod/article-geo2017/blob/master/notebooks/gpr-figure.ipynb
• https://github.com/empymod/article-fdesign/blob/master/notebooks/09-Figure_GPR.ipynb