Merging Method of Images for no-flux Neumann BCs.

Added SE_CLASS1, made box bigger in cylinder unit test s.t. ghost particles of mirrors are within the box.

Changed gradient type from double[dims] to Point<dims, double> also for the temp. grid in the NarrowBand class.

Changed gradient type from double[dims] to Point<dims, double> because preferred by Paraview and because Point has already a function norm, s.t. getting the gradient magnitude doesn't require an extra function.