ENH: improvements in profiling (issue #648)

- include amount of free system memory in profiling, which can give an
  indication of when swapping is about to start

- profilingSummary utility to collect profiling from parallel
  calculations. Collects profiling information from processor
  directories and summarize the time spent and number of calls as (max
  avg min) values.

applications/test/memInfo/Test-memInfo.C

@@ -40,7 +40,7 @@ using namespace Foam;
 int main(int argc, char *argv[])
 {
     const int n = 10000000;
-    const char* const memTags = "peak/size/rss mem: ";
+    const char* const memTags = "peak/size/rss/free mem: ";
 
     memInfo mem;