Summary of changes:
- update main licence to BSD 3-clause and simplify licence boilerplate in the sources
- list of authors and contributors
- generate header file with API functions for Julia bindings
- direct reading of XML UPF-v2 files using pugixml library
- cleanup of tests and unit tests
- introduce real_occupation_matrix setting to enforce Hubbard matrix to be strictly real
- add pseudopotential radial grid cutoff as a parameter instead of a hardcoded 10 a.u. value
- add a code to generate smooth initial magnetisation at a slightly higher initialisation cost
- 'atom' utility is updated to generate high-accuracy LAPW species
- switch to GSL ODE solver for radial equation
- 3-rd order radial derivatives for LAPW functions
- API to set local and non-local occupancy matrices
- API to return major, minor version and revision
- issue a warning for non-optimal k-parallelization
- remove unused input parameters from the schema and move some input parameters to a more logical location
- use strong type for more indices
- iterator over split index
- save magnetisation in JSON output

Fixes:
- sirius_add_hubbard_atom_pair() contained a bug
- init_atoms_to_grid_idx() is now called only in magnetic case
- performance fix for symmetrisation of muffin-tin functions and occupation matrices
- bug-fix in IORA o1 radial integrals
- sirius_set_nonlocal_occupation_matrix(): count spin index starting from 0
- sirius_get_wave_functions(): handle spin index properly 
- linear solver for metals now properly handles k and k+q occupancies
- compute and print DFT total energy and free energy separately