Handle termination of nuXmv/NuSMV if electrod is itself terminated

[grayswandyr]

When electrod is killed from Java, we should kill the underlying solver if it's still running. Until now, we relied upon process sessions but this solution isn't portable. Consider other options and implement one of them:

• do nothing
• remain as now
• remain as now and/or report solver PID to Java
• use another, more portable and robust, approach to kill solver