fix(sparse): zero-init UMFPACK solution vectors (non-deterministic Phi garbage)

[marjohma]

Problem

KIM's Phi_m.dat (and Phi_MA, Phi_m_i/e) intermittently contained uninitialized-memory garbage (e.g. 2.6e+243, 9.6e-315) in a few inner-boundary cells of the localised electrostatic solve — on ~1 in 6 runs. Re-running the same binary on the same inputs gave clean zeros, i.e. non-deterministic.

Root cause

In the SuiteSparse solve routines of QL-Balance/src/base/sparse_mod.f90, the solution vectors (xx/xz for complex, x for real) are ALLOCATEd but never initialized:

ALLOCATE( xx(nrow) )        ! uninitialized
ALLOCATE( xz(nrow) )
...
CALL umf4zsol(sys, xx, xz, bx, bz, numeric, control, info)
b = DCMPLX(xx, xz)          ! copies ALL of xx/xz into the solution

For a field-free localised Poisson solve, the matrix has decoupled/empty rows; umf4zsol leaves those solution entries unwritten, so b = DCMPLX(xx,xz) copies uninitialized heap into the output -> EBdat%Phi -> Phi_m.dat.

Pre-existing latent bug — the solve code is byte-identical across the recent fortnum migration; that migration only perturbed heap/allocation layout, which is why the garbage began surfacing (the archived pre-fortnum output happened to get clean zeros).

Fix

Zero-initialize the solution vectors after allocation in all four SuiteSparse routines (sparse_solve_suitesparse_b1, sparse_solve_suitesparseComplex_b1, and their _b2_loop variants). Decoupled DOFs now come out as 0 — the correct field-free value.

Also delete KIM/src/math/sparse_mod.f90, a stale divergent copy that is no longer compiled (KIM links the shared QL-Balance sparse target; the file is absent from CMakeSources.in). It nearly masked this fix.

Verification

• Compiled in: sparse_mod.f90.o -> libsparse.a -> KIM.x relinked (build log).
• 6/6 clean runs on the AUG 33353/2900 m7 electrostatic case (150/150 grid): no garbage in any Phi field; Phi_MA_ideal matches the archived pre-fortnum reference to <3e-8.
• Correct by construction: explicitly-zeroed memory cannot read as garbage; decoupled DOFs UMFPACK does not write stay 0.