Metering support with time meter (#217)

  * An abstract `meter` class that defines interface for taking a reading, and returning the meter results as a json object.
  * A `time_meter` implementation of the `meter` that measures wall time.
  * To generate metering reports with global information, the global communication policy interfaces were extended to support `gather` and `barrier` operations. These are trivial for serial and dry run policies, and wrap the appropriate MPI calls for the MPI policy.
  * a `meter_manager` type that stores a list of meters was created
    * will also have memory and power meters soon.
  * a meter manager was added to the miniapp and now records startup, model initialization, time stepping and final file io times.