mpi: use mpi attribute consistently as the default MPI implementations

Use the attribute mpi to provide a system wide default MPI
implementation. The default is openmpi (as before).
This now allows for overriding the MPI implentation by using
the overlay mechanism. Build all packages with mpich instead
of the default openmpi can now be achived like this:
self: super:
 {
   mpi = super.mpich;
 }

All derivations that have been using "mpi ? null" to provide optional
building with MPI have been change in the following way to allow for
optional builds with MPI:
{ ...
, mpi
, useMpi ? false
}

1 files changed, 2 insertions, 2 deletions

diff --git a/pkgs/applications/graphics/paraview/default.nix b/pkgs/applications/graphics/paraview/default.nix
index 6e4738dd07c11..7bbff5499f65e 100644
--- a/pkgs/applications/graphics/paraview/default.nix
+++ b/pkgs/applications/graphics/paraview/default.nix
@@ -1,6 +1,6 @@
 { boost, cmake, fetchFromGitHub, ffmpeg, qtbase, qtx11extras,
   qttools, qtxmlpatterns, qtsvg, gdal, gfortran, libXt, makeWrapper,
-  mkDerivation, ninja, openmpi, python3, lib, stdenv, tbb, libGLU, libGL }:
+  mkDerivation, ninja, mpi, python3, lib, stdenv, tbb, libGLU, libGL }:
 
 mkDerivation rec {
   pname = "paraview";
@@ -65,7 +65,7 @@ mkDerivation rec {
   buildInputs = [
     libGLU libGL
     libXt
-    openmpi
+    mpi
     tbb
     boost
     ffmpeg