Timedeltas and arrival/departure ports from MFP export

[j-atkins]

DRAFT

This PR uses the new exported fields from the MFP "Cruise Data" (see #362) to auto-fill the waypoints times using a user-prescribed --start-date and timedeltas from MFP when doing virtualship init --from-mfp.

Also, now that the ports of arrival/departure have been added to the MFP export these are now incorporated into the expedition.yaml, including a new Port model class. These act as special waypoints with times and locations but no instruments.

Further notes

• Refactored virtualship init logic to a new module (cli/_initialise.py). Should help with readability and maintainability now that there is a lot more logic associated with this command, plus is consistent with the structure for other commands (plan and run).
• Now adds comments to the expedition.yaml, marking e.g. # Port of Departure, Waypoint 1, Waypoint 2 and so on. This helps with config readability and is a previous user-feedback request.
• Some of the new logic relies on the MFP export format staying the same and we do not have control over this. I have added checks along the way when ingesting (with instructions to raise an issue if a user believes something has changed). Ideally though we would also unit test for this to catch drift, but I don't know how we would go about this given it requires getting an up to date export from MFP.

TODOs

• Ensure Port waypoints are handled properly when reading expedition.yaml back in plan or run
• Check interaction with 'problems' module
• Adapt plan tool
• Update docs
• More tests

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Closes #362