make Pareto k Inf if it is NA

[topipa]

This PR changes the handling of Pareto k values in edge cases. If Pareto k is NaN or NA, it will be changed to Inf, whereas previously it was done only for NaN. Based on the discussion in #222.

[codecov-commenter]

Codecov Report

Merging #224 (c36fb14) into master (fa4bc97) will not change coverage.
The diff coverage is 100.00%.

❗ Current head c36fb14 differs from pull request most recent head 8b24265. Consider uploading reports for the commit 8b24265 to get more accurate results

@@           Coverage Diff           @@
##           master     #224   +/-   ##
=======================================
  Coverage   93.01%   93.01%           
=======================================
  Files          30       30           
  Lines        2777     2777           
=======================================
  Hits         2583     2583           
  Misses        194      194           

Impacted Files	Coverage Δ
R/gpdfit.R	100.00% <100.00%> (ø)

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[jgabry]

Thanks @topipa!