Add schedule verification step to fetch command

[diogomatoschaves]

• Closes Run verify_schedule at start of fetch command #162

• Moves _get_schedule and _get_ship_config to utils file.

• Refactors validation of schedule and ship_config, which become methods of the Schedule and ShipConfig classes respectively.

• Makes get_instruments_in_schedule a method of the Schedule class

• Adds more tests for the Schedule and ShipConfig classes.

• Moves fetch function to its own module

• Adds schedule verification step at the beginning of the fetch command

• Tests added

[diogomatoschaves]

Hi reviewers, I'm aware that this PR changes more things than the issue it is associated with, but I felt some reorganizing of the code was in order as I was making the changes. If it's too much (in particular the moving of the fetch command function to another module), let me know and I can revert that back.

The commits follow a coherent chronological order, and should be viewed individually to make sense of the changes.

[VeckoTheGecko]

Thanks for the contribution. Just have some changes, mainly around the fixtures/tests and function signatures/readability

[diogomatoschaves]

Hey @VeckoTheGecko I was a bit busy the last 2 weeks, so only now making the requested changes. But here they are! Thanks for the review 👌

[VeckoTheGecko]

All good. Thanks for getting back to this @diogomatoschaves ! Looks good to me now :)

[diogomatoschaves]

There was an error in the tests that had slipped through the cracks. Hopefully now all tests should pass

[VeckoTheGecko]

Thanks!