Multi-materials and derived variable output (AMReX-Codes#2888)

## Summary

Output small plots if only derived variables are specified.
Also, make DeriveFuncFab a std::function<> instead of plain function-pointer.

## Additional background

We have been implementing small-plots for outputing variables at gauges (e.g. pressure at specific gauge locations). We may want to output the derived variable pressure only, and not all state-variables. The if-condition was incorrect in this case.

Further, multi-material simulations require a material index in order to compute derived variables, in addition to existing parameters. Making DeriveFuncFab a std::function is sufficient for our purposes.

Src/Amr/AMReX_Amr.cpp

@@ -910,7 +910,7 @@ Amr::writeSmallPlotFile ()
 
     // Don't continue if we have no variables to plot.
 
-    if (stateSmallPlotVars().size() == 0) {
+    if (stateSmallPlotVars().size() == 0 && deriveSmallPlotVars().size() == 0) {
         return;
     }
 

Src/Amr/AMReX_Derive.H

@@ -84,9 +84,9 @@ extern "C"
                                  const int* level, const int* grid_no) ;
 }
 
-typedef void (*DeriveFuncFab) (const amrex::Box& bx, amrex::FArrayBox& derfab, int dcomp, int ncomp,
-                               const amrex::FArrayBox& datafab, const amrex::Geometry& geomdata,
-                               amrex::Real time, const int* bcrec, int level);
+  typedef std::function<void(const amrex::Box& bx, amrex::FArrayBox& derfab, int dcomp, int ncomp,
+                 const amrex::FArrayBox& datafab, const amrex::Geometry& geomdata,
+                 amrex::Real time, const int* bcrec, int level)> DeriveFuncFab;
 
 class DescriptorList;