Release 0.8.3

SOLVAX 0.8.3 adds additive_preconditioner, a positive weighted combination \(B=\sum_iw_iB_i\) of self-adjoint positive-definite inverse actions. Unlike sequential (multiplicative) composition, the additive sum preserves symmetry, so line, block, and Schwarz-style pieces can be combined for PCG on arrays or arbitrary matching pytrees. See the symmetric additive composition section of Preconditioners.