add function for ES50 calculations:

[aleruete]

https://data-blog.gbif.org/post/exploring-es50-for-gbif/

https://obis.org/indicators/documentation/

ES50 (Hulbert index)
The expected number of marine species in a random sample of 50 individuals (records) is an indicator on marine biodiversity richness.

The ES50 is defined in OBIS as the sum(esi) over all species of the following per species calculation:

when n - ni >= 50 (with n as the total number of records in the cell and ni the total number of records for the ith-species)
esi = 1 - exp(lngamma(n-ni+1) + lngamma(n-50+1) - lngamma(n-ni-50+1) - lngamma(n+1))
when n >= 50
esi = 1
else
esi = NULL
Warning: ES50 assumes that individuals are randomly distributed, the sample size is sufficiently large, the samples are taxonomically similar, and that all of the samples have been taken in the same manner.