Fix #341 PluginConnector

The internal notification std::map caused notifications in non-deterministic order due to internal ordering by plugin pointer which can vary from rank to rank.

The old naming "frequency" was corrected to "period" internally. Interfaces have still to be renamed (#343).

This fix only affects the current `dev` branch. Thanks again to @psychocoderHPC for finding it :)

run time tests:
 - [x] default KHI (16 GPUs)
 - [x] rebased phase space branch (1 GPU, 4 GPUs, 16 GPUs)