[DOC] Add documentation to our classes and structs.

[Irallia]

As correctly noted in review post #68 (review), all member variables and functions still lack any documentation.
This concerns:

• aligned_segment.hpp
• breakend.hpp
• cluster.hpp
• junction.hpp
• variant_record.hpp

A documentation could look like

//! \brief Start positon of the alignment.
int32_t pos;

...

/*! \brief Position associated with this AlignedSegment.
 *
 * \return The position stored as int32_t
 */
int32_t get_reference_start() const;