Uses flatMap instead of map.filter.map

[FrankKair]

Instead of using:

let cmds = cmdsJson.map { Command(jsonObject: $0) }
                .filter { $0 != nil }
                .map { $0! }

We could use flatMap to simplify these operations. It's both more legible and more efficient (1 loop against 3 loops).

let cmds = cmdsJson.flatMap { Command(jsonObject: $0) }

More info: https://developer.apple.com/documentation/swift/sequence/2950916-compactmap

Review Checklist

• [ ] I have responded to reviews and made changes where appropriate.
• [ ] I have tested the code
• [ ] I have updated comments / documentation related to the changes I made.
• [ ] I have rebased my PR branch onto xi-mac/master.

[FrankKair]

I just realized that Xcode 9.2 is being used, so I should use flatMap instead.

[jansol]

While I appreciate the added simplicity, IIRC this line came up earlier and the consensus was to leave it be until Swift 5.0 is released, at which point we'll be interested in bumping the required Xcode version.

[FrankKair]

@jansol Ok, then 😊