Missing documentation for const in Aseba code

Created by: ypiguet-epfl

The Aseba compiler supports constants defined in the source code with keyword const, contrary to what's stated in the documentation. Search for TOKEN_STR_const in files lexer.cpp and parser.cpp. See also issue #244 where const is briefly mentioned, and #159 (closed) which is closed. Suggestion: fix the documentation. If there is a blocking issue, document it in the source code and disable const in released versions of Studio.