Match bounds for observations in frequency estimator classes
Match bounds for observations in frequency estimator classes

Resolves an edge case bug where there are enough observations to pass the
upstream filters of the `freq_est_clipped` class but not enough observations to
pass the `frequency_estimator` class's more restrictive lower bound. This bug
manifests as an attempt to interpolate with a single observation.

This commit resolves the issue by changing the lower bound test of the
`frequency_estimator` class to match the upstream lower bound of the
`freq_est_clipped` class.

nextstrain/augur,nextstrain/augur,nextstrain/augur,blab/nextstrain-augur