convert field energy units to Joule

Convert the output of the Plugin
pls move the common part `* (CELL_VOLUME * float_X(0.5))` just outside of the loop before calculating `globalEnergy`.

e.g.
```c++
energyFieldBReduced *= (CELL_VOLUME * float_X(0.5));
energyFieldEReduced *= (CELL_VOLUME * float_X(0.5));
```