Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/lodash/mean.js

@@ -0,0 +1,22 @@
+var baseMean = require('./_baseMean'),
+    identity = require('./identity');
+
+/**
+ * Computes the mean of the values in `array`.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category Math
+ * @param {Array} array The array to iterate over.
+ * @returns {number} Returns the mean.
+ * @example
+ *
+ * _.mean([4, 2, 8, 6]);
+ * // => 5
+ */
+function mean(array) {
+  return baseMean(array, identity);
+}
+
+module.exports = mean;