Low energy EM updates and bug fix

[lobis]

• Low energy EM updates and bug fix geant4lib#60

This PR fixes the following bug: physics lists options were not being read. Options such as "ARM" of DecayPhysics were not working.

Updates to low energy physics: Previously some options were enabled by default:

UI->ApplyCommand("/process/em/fluo true");
UI->ApplyCommand("/process/em/auger true");
UI->ApplyCommand("/process/em/pixe true");

I found out the option pixe is responsible for a significant increase of simulation time in latest versions of Geant4, due to a lot of very low energy e- being produced. (this is the cause why the 08.Alphas example runs so slow in the pipeline).

The changes in this PR make possible for the user to specify these 3 options as options in the EM physics list. For example:

<physicsList name="G4EmStandardPhysics_option4">
    <option name="pixe" value="true"/>
    <option name="auger" value="false"/>
    <option name="fluo" value="false"/>
</physicsList>

The default value for options fluo and auger is left as true, so the behaviour is the same as previously if not changed. However the option pixe new default value is false, in order to reduce simulation time.

I have modified examples to explictly enabled pixe in order not to modify the validation scripts, however they will be mofied in the future in order to speed them up.