Gemmi

* centred to primitive lattce transformation
* Niggli reduction and Buerger reduction for lattice basis
* added ``UnitCell::is_compatible_with_spacegroup()``
* Topology-related improvements (Keitaro)
* functions ``SMat33::elements()`` and ``change_basis()`` were replaced – each was replaced with two new functions
* more options in gemmi-merge
* more Python bindings
* various fixes, in particular in validation in ``gemmi mtz2cif --depo``.

* reindexing MTZ files,
* tricubic map interpolation,
* extended selection syntax,
* merge_atoms_in_expanded_model() – sort out atoms on special positions after expanding NCS or making bioassembly,
* IT92::normalize() – makes exactly the same scattering coefficients that Refmac uses,
* Mtz::remove_column(),
* understanding triplets with x/N ("h/2+k/2, -h/2+k/2, l"),
* better inference of elements in a PDB file with missing element field,
* Topo::Force was renamed to Topo::Rule,
* various other additions and, most importantly, bug fixes.

Python porting note: CIF parsing may raise `ValueError`; opening a file may rise `IOError`. Previously it was `RuntimeError`.