make stencil_width a static constexpr member of NCIfilter

Author: MaxThevenet

Source/Filter/NCIGodfreyFilter.H

@@ -17,6 +17,8 @@ public:
 
     void getGodfreyCoeffs(godfrey_coeff_set coeff_set_in);
 
+    static constexpr int stencil_width = 4;
+
 private:
 
     godfrey_coeff_set coeff_set;

Source/WarpX.cpp

@@ -633,7 +633,7 @@ WarpX::AllocLevelData (int lev, const BoxArray& ba, const DistributionMapping& d
     int ngz_nonci = (ngz_tmp % 2) ? ngz_tmp+1 : ngz_tmp;  // Always even number
     int ngz;
     if (WarpX::use_fdtd_nci_corr) {
-        int ng = ngz_tmp + 4;
+        int ng = ngz_tmp + NCIGodfreyFilter::stencil_width;
         ngz = (ng % 2) ? ng+1 : ng;
     } else {
         ngz = ngz_nonci;