Add warning to cmake if PKG_KIM=ON but DOWNLOAD_KIM not set & KIM is not found

This can be a scenario where the user has KIM installed but does not have the
environment setup correctly to be found.  The config. step should provide some
warning of this.  Otherwise, it is easy to miss the fact that KIM is being
downloaded and built.

cmake/Modules/Packages/KIM.cmake

@@ -9,6 +9,7 @@ if(PKG_KIM)
   if(KIM-API_FOUND)
     set(DOWNLOAD_KIM_DEFAULT OFF)
   else()
+    message(WARNING "KIM-API package not found.  We will download and build our own")
     set(DOWNLOAD_KIM_DEFAULT ON)
   endif()
   option(DOWNLOAD_KIM "Download KIM-API from OpenKIM instead of using an already installed one" ${DOWNLOAD_KIM_DEFAULT})