Fix the occurrences method for non-species taxa

[tpoisot]

No description provided.

[codecov]

Codecov Report

Merging #30 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   78.37%   78.37%           
=======================================
  Files          10       10           
  Lines         148      148           
=======================================
  Hits          116      116           
  Misses         32       32           

Flag	Coverage Δ
#unittests	78.37% <100.00%> (ø)

Impacted Files	Coverage Δ
src/occurrence.jl	96.29% <100.00%> (ø)

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