Core library

• Add a small selection language a la VMD, allowing to select atoms matching a selection string like "name H and x > 4". This is exposed to C++ with the public Selection class, and to C with the chfl_frame_selection function.
• The C API now provide a direct view into the positions and velocities arrays. This remove the need for copy and separated getter (chfl_frame_{position,velocities}) and setter (chfl_frame_{position,velocities}_set) function. This also force usage of chfl_frame_add_velocities to add velocity data to a frame, and chfl_frame_resize to change the size of the frame.
• It is now possible to provide a callback for logging from the C API. The chfl_log_stdout, chfl_log_silent and chfl_log_callback function where added to the C API.
• Add constants for error codes in C API. The following macro are defined: CHFL_SUCCESS, CHFL_MEMORY_ERROR, CHFL_FILE_ERROR, CHFL_FORMAT_ERROR, CHFL_GENERIC_ERROR, CHFL_CXX_ERROR.

Various cleanup and enhancements:

• Rename all setter function from void xxx(const XXX& value) to void set_xxx(const XXX& value) in C++ API.
• Add the chfl_version function in C API.
• Remove the periodicity handling from UnitCell. It was not implemented in boundaries conditions. The corresponding function where removed from the C API.