bug: missing include guard in `Candapter.h`

[t-bre]

Description

Candapter.h needs an include guard to avoid the problem of double inclusion (as will any other header files).

Ways to do this:

1. #define method:

   #ifndef _HEADER_NAME_H
   #define _HEADER_NAME_H
   
   // header code
   
   #endif

   There are various conventions for how to name the #define, but generally it ends with _H and has one or two underscores at the start.

2. #pragma method:

   #pragma once
   
   // header code

   I find this method cleaner, though it isn't part of the C++ standard so it could make it less portable. That said, I've never used a compiler that doesn't support it.

Expected Behaviour

Include guard exists.