ComputeVCV: Compute the variance-covariance matrix
In bomeara/bmhyd: Comparative Methods for Phylogenetic Networks

Creates a variance-covariance matrix for a network and parameters.

ComputeVCV(
  phy.graph,
  sigma.sq = 1,
  mu = 0,
  bt = 1,
  vh = 0,
  SE = 0,
  measurement.error = 0,
  gamma = 0.5
)

`phy.graph`	An ape::evonet object (a phylogeny stored in phylo format that also includes a reticulation matrix)
`sigma.sq`	Value for sigma squared
`mu`	Value for state at the root
`bt`	Value for beta parameter
`vh`	Value for Vh, the variance that comes from a hybridization event
`SE`	Standard error
`measurement.error`	How much uncertainty there is in tip values; a single number is applied to all taxa, a vector is applied to the corresponding taxa
`gamma`	In a hybridization event, what proportion of the trait comes from the donating parent. 0.5 means half comes from each parent