Add covariance matrix support to the Z-expansion form factor models

[LiangLiu212]

• Add covariance matrix inputs under data/covariances/FormFactor/Axial/ (Deuterium, LQCD v1 and updated, MINERvA elementary target, MINERvA Nature 2x2/4x4) and move the vector ELZExpFF.dat under FormFactor/Vector/; document the files and their sources in a Readme
• Extend ZExpAxialFormFactorModel and ZExpELFormFactorModel to load the covariance matrices used by the reweighting machinery
• Store importfile matrix elements at full precision in AlgConfigPool (std::to_string's fixed 6 decimals can flip a near-singular positive-definite covariance indefinite)
• ZExpELFormFactorModel: hard-code the Gep(0)=1 / Gen(0)=0 charges and read the magnetic moments from the MagnMoments common parameter list instead of per-param_set QEL-G*0 entries
• Add zexp axial param_sets: Deuterium PRD E-matrix default, minerva and lqcd fits from arXiv:2512.14097, minervaNature from Nature 614,48 (lqcd carries the latest-version values with the v1 fit kept commented for reference)