Calculates the indicator value d of species as the product of the
relative frequency and relative average abundance in clusters. Specifically,where:
$p_{i,j} =$ presence/absence (1/0) of species $i$ in
sample $j$;
$x_{i,j}$ = abundance of species $i$ in sample $j$;
$n_c =$ number of samples in cluster $c$;
for cluster $c$ in set $K$;
$$f_{i,c} = {\sum_{j \in c} p_{i,j} \over n_c}$$
$$a_{i,c} = {(\sum_{j \in c} x_{i,j}) / n_c \over \sum_{k=1}^K ((\sum_{j \in k} x_{i,j}) / n_k)}$$
$$d_{i,c} = f_{i,c} \times a_{i,c}$$