correctzerotime crashes for positive-only times

[luisfabib]

When trying to process a time-axis where the first element is already the zero-time, the function crashes.

[luisfabib]

The bug was caused by non-Python indexing in an if-statement (a leftover from the MATLAB-Python porting)

    # Determine location of maximum
    idxmax = np.argmax(V_)

-    if idxmax!=1 and idxmax!=len(V):
+    if idxmax!=0 and idxmax!=len(V_)-1:

Added two new tests to test the that bug is gone.