## Construct the coancestry matrix of an admixture model

### Description

The `n`-by-`n` coancestry matrix `Theta` of admixed individuals is determined by the `n`-by-`k` admixture proportion matrix `Q` and the `k`-by-`k` intermediate subpopulation coancestry matrix `Psi`, given by `Theta = Q %*% Psi %*% t(Q)`. In the more restricted BN-PSD model, `Psi` is a diagonal matrix (with FST values for the intermediate subpopulations along the diagonal, zero values off-diagonal).

### Usage

```coanc_admix(admix_proportions, coanc_subpops)
```

### Arguments

 `admix_proportions` The `n`-by-`k` admixture proportion matrix `coanc_subpops` The intermediate subpopulation coancestry, given either as a `k`-by-`k` matrix (for the complete admixture model), or the length-`k` vector of intermediate subpopulation FST values (for the BN-PSD model; implies zero coancestry between subpopulations), or a scalar FST value shared by all intermediate subpopulations (also implies zero coancestry between subpopulations).

### Details

As a precaution, function stops if both inputs have names and the column names of `admix_proportions` and the names in `coanc_subpops` disagree, which might be because these two matrices are not aligned or there is some other inconsistency.

### Value

The `n`-by-`n` coancestry matrix.

### Examples

```# a trivial case: unadmixed individuals from independent subpopulations
# number of individuals and subpops
n_ind <- 5
# equal Fst for all subpops
coanc_subpops <- 0.2
# diagonal coancestry matryx

# a more complicated admixture model
# number of individuals
n_ind <- 5
# number of intermediate subpops
k_subpops <- 2