Fix species richness for non-integer data (e.g., OTU proportions)

[vmikk]

Hello!

Currently estimate_richness(..., measures = "Observed") works only for integer data:

esp <- transform_sample_counts(esophagus, function(OTU) OTU/sum(OTU) )
estimate_richness(esp, measures = "Observed")

Leads to an error

Error in estimateR.default(newX[, i], ...) : 
  function accepts only integers (counts)

I propose to change estimateR-function for vegan::specnumber which works fine for non-integer data (e.g., OTU proportions).

This commit does not change column names and the order of columns in estimate_richness results, so it shouldn't impact any dependent functions.

With best regards,
Vladimir