<code>estimate_liability</code> has been updated to allow for a higher
degree of flexibility. This means it is up to the user to provide the
familial relationship, and <code>construct_covmat</code> creates the
corresponding covariance matrix based on the heritability and expected
genetic overlap between two individuals.</p>
<p><code>construct_covmat</code> defaults to a family structure with
both parents, one sibling, and the paternal and maternal grandparents.
The input format for <code>construct_covmat</c