Add const in CharCodeToUnicode

I submit this merge request to update some functions that can be transformed to const.

As we discussed on IRC, when we created a charCodeToUnicode structure we can't use non const functions.

In CharCodeToUnicode.h

Functions updated:

CharCode getLength() const
int mapToUnicode(CharCode c, Unicode **u) const

In CharCodeToUnicode.cc

Function updated:

int mapToUnicode(CharCode c, Unicode **u)