Add transect update monitoring flow

[LOCEANlloydizard]

EDIT

following the workflow discussed in #163, this PR adds event-based monitoring of transect start/end updates, and builds toward the event-driven processing described in #159.

we add:

• a generic file/directory watcher using watchdog
• a raw-file watcher that emits a Prefect event when matching .raw files arrive/update
• a transect-specific watcher that emits a Prefect event when the transect CSV is updated
• a transect_update flow triggered by this event, including:
  • ignoring incomplete/open transects
  • detection of new/updated transect segments using a snapshot of the previous CSV
  • identification of existing Sv files that overlap each updated transect time range

The Sv lookup currently uses Sv_files.csv from the existing raw2Sv workflow. This can later be replaced by the database-backed lookup as that functionality is integrated (#141)

A small companion example showing how to configure and launch the watchers is included in the corresponding echodataflow-recipes PR: echostack-org/echodataflow-recipes#5

Refs: #163, #159

[leewujung]

Oh sorry @LOCEANlloydizard I turned on the auto RTD build, but until #170 is merged that would continue to fail. 😅

[LOCEANlloydizard]

oh cheers! yes i saw the fails and then saw the #170.. it all make sense now! 😅

[LOCEANlloydizard]

@leewujung i updated this PR to also include the generic raw-directory monitoring discussed in #159, together with the transect monitoring from #163! I think its good for review, cheers!