Fix gallery example

examples/plot_calculator.py

-"""
-How to use the Calculator
-=========================
-
-The following example shows how to use the `Calculator` class.
-"""
-from km3net_testdata import Calculator
-
-#####################################################
-# Initialising a Calculator
-# -------------------------
-# Initialising is a simple as this:
-
-calc = Calculator()
-
-#####################################################
-# Adding numbers
-# ~~~~~~~~~~~~~~
-#
-# You can add two numbers by calling the `.add(a, b)` method:
-
-print(calc.add(5, 23))

examples/plot_usage.py

+"""
+How to use the package
+=======================
+
+The following example shows how to use the `Calculator` class.
+"""
+from km3net_testdata import data_path
+
+#####################################################
+# Getting a filename
+# ------------------
+# To get the filename of a sample file:
+
+filename = data_path("km3net_online.root")
+print(filename)