Various platform agent updates

- additional preparations for the CGSN case (still more internal than involving integration with platform driver)
- removal of notes.txt files in the code as requested. (here's the wiki now hosting those notes: https://confluence.oceanobservatories.org/display/CIDev/Platform+agent-driver+dev+notes)
- keep the reported NTP-timestamped engineering data (per recent email discussion)
- remove interface operations related with "external event dispatch" in the platform agent; instead these are internal actions as part of the connection/disconnection of the RSN platform driver.
- overwrite on_init in PlatformAgent to perform a graceful termination when the current state is not UNINITIALIZED (which can be triggered from the UI via the "Stop Platform Agent" button regardless of the current state, for example).