Time can not be zero

[LebedevRI]

When running big_o.infer_big_o_class(x, y) on data
that may happen to (unintentionally) have zeros in y,
there are currently run-time errors.

It would be good to mention in the docs,
that times can't be zeros,
and provide a workaround example:

import sys

y = <...>

# "Time" can not be zero.
y = [ max(e, sys.float_info.epsilon) for e in y ]

best, fitted = big_o.infer_big_o_class(x, y)