Extend calculate_PD() to include other PD metrics

[LBreugelmans]

A list is needed of other PD metrics that can be calculated within our workflow (using the datacube and phylogenetic tree as input).

The calculate_PD() function needs to have a parameter which can be a list of multiple metrics. All specified metrics should be calculated and added to the cube.
Eg.

pd_metrics<- c(“Faith”, “ED”, “MNTD”)
calculate_PD(tree, cube, pd_metrics)