add sqrt(4pi) factor to magnetic field unit conversion

[BenWibking]

Internally, AthenaPK (and almost all MHD codes) use Heaviside-Lorentz units, where the magnetic field is transformed from $B \rightarrow B / \sqrt{4 \pi}$. (See also the note in the Castro documentation about this.)

So when converting from CGS-Gaussian units to code units, it is necessary to divide by $\sqrt{4 \pi}$ (in addition to the base dimensional factors).

When converting from code units to CGS-Gaussian units, it is necessary to multiply by $\sqrt{4 \pi}$ (in addition to the base dimensional factors).

[pgrete]

I also checked the code neither code_magnetic_cgs nor gauss nor microgauss (which use the former) were used anywhere in the code, so I don't this this fix has (or had) any imminent impact.