Compare division by zero strategies

Division by zero is unavoidable in a vectorized implementation of `p_norm` since zeros appear along the diagonal of the matrix. There are (at least) two strategies to overcome this:
1. Catch the `numpy` division by zero errors and convert the output to `nan`.
2. Add some small epsilon along the diagonal so that we no longer have zeros.

Strategy 1. is currently implemented.
https://github.com/dwavesystems/minorminer/blob/3c0331f95b64c9513f76ad95e686ea05391ea6f4/minorminer/layout/layout.py#L125
We should compare run-time differences and layout quality differences between the two strategies.