Bugfix `get_transpiration()`

[dschlaep]

get_transpiration() and get_evaporation() now check if column "tran_cm" of slot "AET" exists before using it

[codecov[bot]]

Codecov Report

Merging #231 (8b2b7dd) into main (bd7552e) will increase coverage by 0.04%. The diff coverage is 100.00%.

:exclamation: Current head 8b2b7dd differs from pull request most recent head 5265c5b. Consider uploading reports for the commit 5265c5b to get more accurate results

@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
+ Coverage   55.05%   55.09%   +0.04%     
==========================================
  Files          42       42              
  Lines        6479     6485       +6     
==========================================
+ Hits         3567     3573       +6     
  Misses       2912     2912              

Impacted Files	Coverage Δ
R/sw_OutputDerived_Functions.R	82.60% <100.00%> (+0.95%)	:arrow_up: