Add a transform_forecasts() function

[nikosbosse]

This proposal is very similar to #271, with the only difference that the log_shift() function has no negative_to_zero argument. This seemed somewhat controversial, so this PR introduces the main transform_forecasts() function and we can think about adding the function argument at a later time.

[codecov]

Codecov Report

Merging #278 (dcffbf4) into master (66487aa) will decrease coverage by 0.27%.
The diff coverage is 81.57%.

❗ Current head dcffbf4 differs from pull request most recent head d2b3132. Consider uploading reports for the commit d2b3132 to get more accurate results

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
- Coverage   90.82%   90.55%   -0.27%     
==========================================
  Files          21       22       +1     
  Lines        1286     1324      +38     
==========================================
+ Hits         1168     1199      +31     
- Misses        118      125       +7     

Impacted Files	Coverage Δ
R/convenience-functions.R	81.57% <81.57%> (ø)

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