mpiP v3.1.0 release

We are pleased to announce the release of version 3.1.0 of the mpiP lightweight profiling library for MPI applications.

Release v3.1 addresses the following issues:

* Configure defaults to the optimal platform timer, but the timer can be specified with configure options.
* Support for rts_get_timebase on BG/L.
* Runtime flags can be used to specify that report generation be done by a single collector process (default) or by using a small fixed amount of memory and MPI collectives. The single collector process approach tends to be faster while the low-memory approach is useful for systems where memory availability may be a concern.