Implement $drop(hard = TRUE)

[iferres]

Implement an option to completely remove an organisms from the dataset. This would allow to then add metadata to available organisms without NAs needed for dropped ones. In the current implementation user must add metadata for all organisms, included the hidden ones. This makes it unnecessarily complicated if those organisms are garbage.

Suggested implementation:

p$drop( x = 1:4, hard = TRUE)

I now think default should be hard = FALSE (current behavior), but not sure.