Refactor API layer, add missing models, etc.

[yeahphil]

• Make an API group/dir and move API related files to it
• Mark all funcs in AWApiReachHelper public/private
• Delete AWApiArticleHelper, which had been replaced with graphql calls
• Add an API singleton, with funcs to proxy all public reach calls to AWApiReachHelper
• Use API to make reach API calls
• Remove AWApiLinks.swift (url globals), API passes down URLs to its helper
• Whoops, API/AWApiReachHelper.fetchReachesByRegion had no callers
• Change API funcs from callback/callbackError handlers to a completion handler with an optional Error
• Rename a couple API funcs
• Move GraphQL stuff into API group
• API.updateReaches takes Int16 ids and stringifies them
• API.updateReachDetail takes an Int16 id
• Add proxies for all the public funcs in AWGQLApiHelper
• Take away AWGWLApiHelper.shared and run its calls through API
• Run AWGQLArticleApiHelper through API
• Change Reach.id from Int16 to Int
• Rename API graphql-based calls a little and change id's passed as String to Int
• Whoops, fix a refactoring mistake: dropped a bunch of params from postPhoto()
• Refactor getAccidentsForReach() and the stuff that uses it
• Change AWGQLApiHelper's error callback type from (Error?, String?) to (Error)
• Delete GQLError, now unused
• Refactor API.getAlertsForReach() and the stuff that uses it
• Update API.postAlert and add alert controller to use Alert type
• Update API.getGaugeObservations, adding GaugeObservation type and refactoring as needed
• Convert API.postGaugeObservation to completion style callback
• Add a Photo model. Refactor API.getPhotos to use a completion callback returning [Photo]
• Now that there's a Photo model, have GaugeObservation use it for its photos
• Refactor API.postPhoto, returning a Photo or Error
• Small cleanup: unnest an if-let by changing to guard
• Move a surprise API call (get gauge graph) that was made from a view controller into API
• Create a type GaugeDataPoint and use it to represent gauge data
• GaugeDataPoint: change updated (a Double) to date (a Date)
• Clarify that some model classes are only used internally by AWApiReachHelper
• Delete Article, unused
• More model file cleanup
• API.getGauges cleanup
• Convert API.getMetrics to return [Metric] instead of [String: String]
• Remove many print statements from AWGQLApiHelper, formatting and some error handling cleanup
• Reach API: extract managedObjectContext and main context merging rather than repeating in every method
• Remove DispatchGroup stuff from AWApiReachHelper, extract private context creation to a private func
• Add a ReachUpdater wrapper around API reach methods and call it when reach updates need to be made
• Remove updateAllReachDistances() from API, AWApiReachHelper and move to ReachUpdater
• Move persistence bits for ReachUpdater.updateReaches(regionCodes:) into ReachUpdater
• Move persistence for updateReaches(reachIds:completion:) into ReachUpdater
• Remove ReachUpdater.updateAllReaches(...), equivalent to updateReaches(regionCodes: Region.all.map { $0.code })
• Move persistence stuff for updateReachDetail into ReachUpdater
• Remove AWApiReachHelper dependency on CoreData
• Move persistence-related parts of AWGQLArticleApiHelper into an ArticleUpdater
• Update API
• Fixes and improvements to AddRiverFlowTableViewController
• Oops -- I was mistaken, this works fine (river flows cell types)
• Oops -- I changed the condition for making a network call in RunsListViewController, change back