Test a triad

Dobereiner's rule says that the middle mass of a triad is close to the average of the other two. You will write the average, a test that allows $2\ \%$, and list the groups of three that pass.

The idea. A triad passes if $\dfrac{|\text{average} - \text{middle}|}{\text{middle}} \times 100 < 2$.

Your tasks.

1. In average, return the average of the first and last masses.
2. In is_triad, return True if the middle mass is within $2\ \%$ of that average.
3. Fill triads with the names in candidates that pass.