Only use MPI if explicitly enabled

Running with MPI on a regular workstation will probably not increase
performance, so this should be disabled as default.

CMakeLists.txt

@@ -97,8 +97,13 @@ include (UseOptimization)
 # dependencies, in case they alter the list of warnings
 include (UseWarnings)
 
+# parallel computing must be explicitly enabled
+option (USE_MPI "Use Message Passing Interface for parallel computing" OFF)
+if (NOT USE_MPI)
+	set (CMAKE_DISABLE_FIND_PACKAGE_MPI TRUE)
+endif (NOT USE_MPI)
+
 ### --- begin opm-core specific --- ###
-# parallel programming
 include (UseOpenMP)
 find_openmp (${project})
 ### --- end opm-core specific --- ###