Add cache for time provider

The improvement is surely negligible, perhaps harming performance, so more of a gimmick

src/main/java/eu/m724/wtapi/provider/twilight/CacheableTwilightTimeProvider.java

@@ -0,0 +1,40 @@
+package eu.m724.wtapi.provider.twilight;
+
+import eu.m724.wtapi.object.Coordinates;
+import eu.m724.wtapi.object.Twilight;
+
+import java.time.LocalDate;
+
+// TODO reconsider if this is necessary and doesn't actually harm performance
+/**
+ * A {@link TwilightTimeProvider} that can cache some calculations to be faster
+ *
+ * @param <T> An implementation of {@link TwilightTimeCache} where you store whatever you need
+ */
+public abstract class CacheableTwilightTimeProvider<T extends TwilightTimeCache> extends TwilightTimeProvider {
+    /**
+     * Calculates sunrise and sunset for provided coordinates
+     *
+     * @param cache {@link T}, implementation of {@link TwilightTimeCache}
+     * @param coordinates Coordinates of the observer
+     * @return {@link Twilight}
+     * @see #initializeCache(LocalDate)
+     */
+    public abstract Twilight calculateTwilightTime(T cache, Coordinates coordinates);
+
+    /**
+     * Initializes cache for provided date
+     *
+     * @param date The UTC date
+     * @return ready to use cache
+     */
+    public abstract T initializeCache(LocalDate date);
+
+    @Override
+    public Twilight calculateTwilightTime(LocalDate date, Coordinates coordinates) {
+        return calculateTwilightTime(
+                initializeCache(date),
+                coordinates
+        );
+    }
+}

src/main/java/eu/m724/wtapi/provider/twilight/TwilightTimeCache.java

@@ -0,0 +1,4 @@
+package eu.m724.wtapi.provider.twilight;
+
+public interface TwilightTimeCache {
+}