* Our periodicity is kernel independant but is not a real/usual periodicity.* We simply repeat the simulation box a high number of times which means that if* needed the simulation box can be repeated a million times in each direction.* In order to enable this feature nothing special is needed, any ScalFMM kernel can* be used with this periodicity.* Moreover the usual FMM garantees apply here because we are doing nothing more than a FMM.* Please refer to the Periodic example to know more and test the degree of repetition based on * the "-per" parameter.