Revert "Changed implementation of split."

This reverts commit c79d0faa41be0fc00f3e911fd83ad54774062d01.
Moved out val chunkSize to top of file.
Changed impl. of split to split in chunks of chunkSize.
Changed spec. of split to split in chunks of chunkSize.
Changed PRE of split.
Changed PRE and POST in specs. of numToLetter and letterToNum.
Changed POST and EXAMPLE in spec. of preprocess.
Changed implementation of enDecrypt to not do double work.