chemfiles/chemfiles

Add a public Residue class to C++ and C API to represent residue data.
Residues are groups of atoms bonded together, which may or may not correspond
to molecules; and are often used for bio-molecules.
Add the resname and resid selector, to select atoms based on their
residue.
Account for the difference between the atom name ("H1") and atom type ("H")
in some formats (PDB, TNG, ...). This introduces the Atom::type member
function and the chfl_atom_type C API function.
Add the type selector, to select atoms based on their type.
Add "Frame::add_atom" function to add an atom and the corresponding position
(and velocity) data to a frame, and the C API chfl_frame_add_atom function.
Rename UnitCell::type to UnitCell::shape. This also affect
chfl_cell_shape_t, chfl_cell_shape, and chfl_cell_set_shape.
All the floating point data uses doubles instead of floats. This concerns
atomic data, positions and velocities.
Add "Selection::string" function and the corresponding chfl_selection_string
to get the string used to build a selection.
Selection variables uses the #3 syntax instead of the $3 syntax to allow
passing selection string as shell arguments.
Add Frame::remove and chfl_frame_remove to remove an atom in a frame.
Allow to use chemfiles with Cmake find_package.

Add read support for TNG files, an new portable and compressed binary format
used by GROMACS.

All the integers at C boundary have a fixed size, most of the time using
uint64_t.
Add missing chfl_topology_resize function to C API.
C API functions taking three lengths/angles now take a double[3] parameter
instead.
Rename chfl_topology_are_linked to chfl_topology_residues_linked.
Rename chfl_topology_append to chfl_topology_add_atom.
Remove chfl_strerror, as it is redundant with chfl_last_error.
Merge chfl_trajectory_set_topology_file and chfl_trajectory_set_topology_with_format
into chfl_trajectory_topology_file.
The chfl_frame function no longer take the frame size as argument. It always
creates an empty frame, that you can resize using chfl_frame_resize.
chfl_selection_evalutate was a typo, it is renamed to chfl_selection_evaluate.

Remove the Atom::type enum from C and C++ API.
Remove the Trajectory::sync and the chfl_trajectory_sync functions.
To ensure that all content of a file is written to the disk, the user need to
close it.
Remove the Logger and all the chfl_log* functions. Instead, the users can
use chemfiles::set_warning_callback or chfl_set_warning_callback to set a
global callback to call on warnings events.