Negative j_hot currents interpolated incorrectly in isotropic mode

[hoppe93]

In isotropic mode, it has been discovered that when the hot current density j_hot is negative, the hot current density term incorrectly yields zero hot current. The reason for this is line 339 in src/Equations/Fluid/HotTailCurrentDensityFromDistributionFunction.cpp which compares the two hot current components j_1 and j_2 (which can be either positive or negative) directly with a positive number (sqrt(double_min); in order to avoid over-/underflow).