fix: Relates to #9. Reuse calculation of average block time

Issues:

* Unable to import `blockAverage` into Chain.ts or Node.ts due to Issue #21.
* TypeScript says `[ts] Module '"/Users/Me/code/blockchain/clones/paritytech/dotstats/packages/common/build/index"' has no exported member 'blockAverage'.`

packages/common/src/block.ts

@@ -0,0 +1,19 @@
+import { Maybe, Types } from '@dotstats/common';
+
+export function blockAverage(blockTimes: Array<number>): Maybe<Types.Milliseconds> {
+  let count = 0;
+  let sum = 0;
+
+  for (const time of blockTimes) {
+    if (time) {
+      count += 1;
+      sum += time;
+    }
+  }
+
+  if (count === 0) {
+    return 0 as Types.Milliseconds;
+  }
+
+  return (sum / count) as Types.Milliseconds;
+}

packages/common/src/index.ts

@@ -1,5 +1,6 @@
 export * from './helpers';
 export * from './id';
+export * from './block';
 
 import * as Types from './types';
 import * as FeedMessage from './feed';