Set constants to correct SI (2019) value

The constant mu0, and by extension also eps0 and Z0, is slightly wrongly defined. By the 2019 SI standard, \mu_0 \approx 1.25663706212 \cdot 10^{6} H m^{-1} \neq 4\pi \cdot 10^{-7} H m^{-1}.

This MR corrects this, with eps0 and Z0 still given by their relation to mu0. Additionally, the MR ensures that the value defined in IW2D.h is not overwritten locally in other C++ source files.

While the code in the MR does not depend on !16 (merged), the results presented below and the updated reference data files were calculated with both fixes implemented, in order to not have to update the files twice.

Solves #17 (closed).