Fix spelling in comments and documentation using codespell

Apply the utility from https://github.com/codespell-project/codespell/
to fix spelling issues in the headers under both include and interface
directories and add a file with a couple of exceptions.

The exact command line used was:

    $ codespell -w -I misc/scripts/codespell.ignore -i 3 in*

misc/scripts/codespell.ignore

@@ -0,0 +1,6 @@
+convertor
+malcom
+ot
+othere
+seh
+ser