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.