compute_hr doesn't check that QRS indices are strictly increasing #602

compute_hr assumes qrs_inds is strictly increasing, but doesn't check it. If two beats have the same index, the RR interval is 0:

import numpy as np
from wfdb import processing

processing.compute_hr(20, [2, 8, 8, 15], fs=1)
# ZeroDivisionError: float division by zero

processing.compute_hr(20, np.array([2, 8, 8, 15]), fs=1)
# RuntimeWarning: divide by zero, and the output contains inf

Beats that go backwards (e.g. [2, 8, 5, 15]) give a negative interval and a negative heart rate (-20 bpm in this example) without any error.

This can happen if annotations are combined from several sources or include non-beat annotations at the same sample (see #185).

compute_hr could check that qrs_inds is strictly increasing and raise a clear ValueError if not. #601 makes the last interval count as well, so a repeated last beat now hits the same problem.