Add power meter and refactor meter interfaces.

Fixes #190.

The final piece in the metering features.

* Add a `power_meter` which currently records energy used on each node of a Cray XC{30,40,50} systems, which all have built in `pm_counters` interface to power measurement.
* Add information about which node each MPI rank runs on to the metering output in `meters.json`, which is needed to analyse energy recordings, which are per node, not per MPI rank.
* Refactor collation of measurements: now the responsibility of the meter manager.
* Add support for `gather` with `std::string` to the global communication policy, which required a back end MPI implementation and corresponding unit test.
* Add `src/util/config.hpp` that populate the `nest::mc::config` namespace with `constexpr bool` flags describing system or environment capabilities.

src/util/power.hpp

+#pragma once
+
+#include <cstdint>
+
+namespace nest {
+namespace mc {
+namespace util {
+
+// Energy in Joules (J)
+using energy_size_type = std::uint64_t;
+
+// Returns negative value if unable to read energy
+energy_size_type energy();
+
+} // namespace util
+} // namespace mc
+} // namespace nest

tests/global_communication/test_mpi_gather_all.cpp

@@ -97,4 +97,52 @@ TEST(mpi, gather_all_with_partition) {
     EXPECT_EQ(expected_divisions, gathered.partition());
 }
 
+TEST(mpi, gather_string) {
+    using policy = mpi_global_policy;
+
+    int id = policy::id();
+
+    // Make a string of variable length, with the character
+    // in the string distrubuted as follows
+    // rank string
+    //  0   a
+    //  1   bb
+    //  2   ccc
+    //  3   dddd
+    //   ...
+    // 25   zzzz...zzz   (26 times z)
+    // 26   aaaa...aaaa  (27 times a)
+    auto make_string = [](int id) {
+        return std::string(id+1, 'a'+char(id%26));};
+
+    auto s = make_string(id);
+
+    auto gathered = mpi::gather(s, 0);
+
+    if (!id) {
+        ASSERT_TRUE(policy::size()==(int)gathered.size());
+        for (std::size_t i=0; i<gathered.size(); ++i) {
+            EXPECT_EQ(make_string(i), gathered[i]);
+        }
+    }
+}
+
+TEST(mpi, gather) {
+    using policy = mpi_global_policy;
+
+    int id = policy::id();
+
+    auto gathered = mpi::gather(id, 0);
+
+    if (!id) {
+        ASSERT_TRUE(policy::size()==(int)gathered.size());
+        for (std::size_t i=0; i<gathered.size(); ++i) {
+            EXPECT_EQ(int(i), gathered[i]);
+        }
+    }
+    else {
+        EXPECT_EQ(0u, gathered.size());
+    }
+}
+
 #endif // NMC_HAVE_MPI