[wien2k] Pure-Python case.symqmc generator (first dmftproj port step)

[krystophny]

Part of #7 (dmftproj Fortran-to-Python port).

First step of porting the dmftproj Fortran to Python: a pure-Python generator for case.symqmc (the correlated-shell symmetry matrices).

triqs_dftkit.wien2k.symqmc.write_symqmc(case) reads case.dmftsym + case.indmftpr + case.struct and writes case.symqmc, reproducing the dmftproj Fortran construction (Wigner D, orbital and spinor time-reversal operators, angular-harmonics basis transform, spin-1/2 phase blocks). It is a full replacement of the symqmc-writing, covering every path dmftproj emits:

• non-mixing spin-diagonal bases (complex, cubic): the up/up block scaled by the +-(a+g)/2 phase, with the orbital time-reversal operator on the magnetic operations;
• spin-mixing fromfile bases (the |j,m_j> basis): the full 2(2l+1) spinor representation P spinrot P^dag with the spinor time-reversal -i sigma_y (x) T on the magnetic operations;
• l=0 (s) shells: the 2x2 spin phase block.

This is the first of the "tail-backward" series that incrementally replaces fortran/dmftproj with Python, each step proven against the Fortran output via the SOC golden test (#4).

While validating the mixing path I found dmftproj reads the fromfile basis in single precision (a CMPLX cast without KIND=8) and through a 250-column line cap, so its case.symqmc there carries a ~1e-7 error; the Python generator is full double precision. That dmftproj bug is fixed separately in #6.

Verification

Validated against the dmftproj Fortran output on CaOs2 (16 operations, 8 time-reversal) for all three paths:

non-mixing (cubic): max|py - fortran| = 6.8e-8,  unitarity dev = 4e-16
mixing (|j,m_j>):   max|py - fortran| = 6.0e-8,  unitarity dev = 9e-9   (8-digit input floor)
l=0 (s):            max|py - fortran| = 4.4e-8,  unitarity dev = 0

The residuals are dmftproj's own limited precision (8-digit cubic templates, single-precision fromfile); the Python construction is exact (machine-precision unitarity). With #6 applied, the mixing path matches dmftproj to 1.9e-14.

End to end, the cubic converter HDF5 is identical (h5diff). Tests: the cubic path reuses the SOC golden h5; the mixing and l=0 paths compare to the dmftproj matrix data (single precision, ~26 kB) and assert unitarity. Built against TRIQS with Build_Tests=ON: 4/4 wien2k tests pass.

[krystophny]

Accuracy: reproduces the released (single-precision) dmftproj case.symqmc to 6.8e-8. With the Fortran precision fix #14 + exact Python #12 this becomes 1.5e-14 (machine precision).