add CMake support for fetching external potential files

cmake/Modules/LAMMPSUtils.cmake

@@ -85,3 +85,20 @@ function(GenerateBinaryHeader varname outfile files)
         file(APPEND ${outfile} "const unsigned int ${varname}_size = sizeof(${varname});\n")
     endforeach()
 endfunction(GenerateBinaryHeader)
+
+# fetch missing potential files
+function(FetchPotentials pkgfolder potfolder)
+  if (EXISTS "${pkgfolder}/potentials.txt")
+    set(LAMMPS_POTENTIALS_URL "https://download.lammps.org/potentials")
+    file(STRINGS "${pkgfolder}/potentials.txt" linelist REGEX "^[^#].")
+    foreach(line ${linelist})
+      string(FIND ${line} " " blank)
+      math(EXPR plusone "${blank}+1")
+      string(SUBSTRING ${line} 0 ${blank} pot)
+      string(SUBSTRING ${line} ${plusone} -1 sum)
+      message("Checking whether to download external potential ${pot} from ${LAMMPS_POTENTIALS_URL}")
+      file(DOWNLOAD "${LAMMPS_POTENTIALS_URL}/${pot}.${sum}" "${LAMMPS_POTENTIALS_DIR}/${pot}"
+        EXPECTED_HASH MD5=${sum} SHOW_PROGRESS)
+    endforeach()
+  endif()
+endfunction(FetchPotentials)