Fix spell check false positive by ignoring word (#497)

In the latest release of the codespell tool used for automated spell checking of the files of this project, the word "clen" was added to the codespell misspelled words dictionary as a misspelling of "clean".

This caused a false detection of a struct member name `clen` as a misspelling, resulting in a failing spell check result. Since the occurrence of this name is correct and intended in this project, the false positive is resolved by configuring codespell to ignore the problematic word.

.codespellrc

 # See: https://github.com/codespell-project/codespell#using-a-config-file
 [codespell]
 # In the event of a false positive, add the problematic word, in all lowercase, to a comma-separated list here:
-ignore-words-list = hart,pullrequest
+ignore-words-list = clen,hart,pullrequest
 builtin = clear
 check-filenames =
 check-hidden =