Create a MiniGo library product. (#141)

* Create a MiniGo library product for use with Colab import.
* Rename `MiniGoMain` to `MiniGoDemo`.

Author: rxwei

Package.swift

@@ -9,7 +9,8 @@ let package = Package(
         .executable(name: "MNIST", targets: ["MNIST"]),
         .executable(name: "CIFAR", targets: ["CIFAR"]),
         .executable(name: "ResNet", targets: ["ResNet"]),
-        .executable(name: "MiniGo", targets: ["MiniGo", "MiniGoMain"]),
+        .executable(name: "MiniGoDemo", targets: ["MiniGoDemo"]),
+        .library(name: "MiniGo", targets: ["MiniGo"]),
     ],
     targets: [
         .target(name: "Autoencoder", path: "Autoencoder"),
@@ -19,7 +20,8 @@ let package = Package(
         .target(name: "Gym-CartPole", path: "Gym/CartPole"),
         .target(name: "MNIST", path: "MNIST"),
         .target(name: "MiniGo", path: "MiniGo", exclude: ["main.swift"]),
-        .target(name: "MiniGoMain", dependencies: ["MiniGo"], path: "MiniGo", sources: ["main.swift"]),
+        .target(name: "MiniGoDemo", dependencies: ["MiniGo"], path: "MiniGo",
+                sources: ["main.swift"]),
         .testTarget(name: "MiniGoTests", dependencies: ["MiniGo"]),
         .target(name: "ResNet", path: "ResNet"),
         .target(name: "Transformer", path: "Transformer"),