Non-zero estimate_u_values

[samnlindsay]

Closes #161

Trivial fix to ensure estimate_u_probs returns at least 1/target_rows for all u values (equivalent to 1 comparison found for that gamma level)

Warning to users on manually setting any m_probabilities or u_probabilities to zero.

[RobinL]

Nice. Can you:
(1) increment splink version in toml file
(2) expand the comment in the code to include a hyperlink to the issue: #161