arguments: <code>cases</code>, <code>incidence</code>,
<code>population</code>, <code>incidence_denominator</code>,
<code>time</code> and <code>time_interval</code>. As default both
<code><a href="../reference/seasonal_onset.html">seasonal_onset()</a></code> and <code><a href="../reference/seasonal_burden_levels.html">seasonal_burden_levels()</a></code>
use <code>cases</code>, but if <code>incidence</code> is in the
<code>tsd</code> object the output will be in <code>incidence</code>
instead. If <code>popul