add encoder spec

- mostly as example

src/mol-io/common/_spec/encoder.spec.ts

+import { ArrayEncoding } from '../binary-cif/array-encoder';
+import { decode } from '../binary-cif/decoder';
+
+const E = ArrayEncoding;
+
+test('fixedPoint2', async () => {
+    const fixedPoint2 = E.by(E.fixedPoint(100)).and(E.delta).and(E.integerPacking);
+
+    const x = [1.092, 1.960, 0.666, 0.480, 1.267];
+    const y = [7.428, 7.026, 6.851, 7.524, 8.333];
+    const z = [26.270, 26.561, 25.573, 27.055, 25.881];
+
+    const xEnc = fixedPoint2.encode(new Float32Array(x));
+    const yEnc = fixedPoint2.encode(new Float32Array(y));
+    const zEnc = fixedPoint2.encode(new Float32Array(z));
+
+    expect(xEnc.data.length).toEqual(6);
+    expect(yEnc.data.length).toEqual(5);
+    expect(zEnc.data.length).toEqual(6);
+
+    const xDec = decode(xEnc);
+    const yDec = decode(yEnc);
+    const zDec = decode(zEnc);
+
+    x.forEach((a, i) => expect(xDec[i]).toBeCloseTo(a, 2));
+    y.forEach((a, i) => expect(yDec[i]).toBeCloseTo(a, 2));
+    z.forEach((a, i) => expect(zDec[i]).toBeCloseTo(a, 2));
+});