Add spark notebook tests and change examples

* Test added for all kernels
* Same examples as provided in the documentation (`specifics.md`)
* Used the same use case for all examples: sum of the first 100 whole numbers

Note: I've not automatically tested `local_sparklyr.ipynb` since it creates by default the `metastore_db` dir and the `derby.log` file in the working directory. Since I mount it in `RO` it's not working. I'm struggling to set it elsewhere...

all-spark-notebook/test/data/local_spylon.ipynb

@@ -0,0 +1,63 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": 6,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "%%init_spark\n",
+    "# Spark session & context\n",
+    "launcher.master = \"local\"\n",
+    "launcher.conf.spark.executor.cores = 1"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 7,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[8] at parallelize at <console>:28\n",
+       "res4: Double = 5050.0\n"
+      ]
+     },
+     "execution_count": 7,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "// Sum of the first 100 whole numbers\n",
+    "val rdd = sc.parallelize(0 to 100)\n",
+    "rdd.sum()\n",
+    "// 5050"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "spylon-kernel",
+   "language": "scala",
+   "name": "spylon-kernel"
+  },
+  "language_info": {
+   "codemirror_mode": "text/x-scala",
+   "file_extension": ".scala",
+   "help_links": [
+    {
+     "text": "MetaKernel Magics",
+     "url": "https://metakernel.readthedocs.io/en/latest/source/README.html"
+    }
+   ],
+   "mimetype": "text/x-scala",
+   "name": "scala",
+   "pygments_lexer": "scala",
+   "version": "0.4.1"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 4
+}