PySCF v2.6.0 release

Added
- SMD and PCM solvent model
- Nuclear Hessian for SMD and PCM solvent models
Improved
- DFT-D3 and DFT-D4 interfaces. Supporting assigning the acronym such as
  WB97M-D3BJ, WB97M-D4 to the xc attribute.
- Optimized density fitting performance.
- Memory usage for RPA.
- FCI coefficients transformation for more than 64 orbitals.
- Multi-grid DFT performance greatly optimized.
Fixes
- CASCI/CASSCF calculations when mixing C1 symmetry and symmetry=False .
- "atom" initial guess for ECP with super-heavy atoms.
- Complex conjugation issues in single k-point JK-build in PBC DF.
- CCSD(T) for complex orbitals.
- Dipole moment in SFX2C.
- High-order XC derivatives.
- Integer overflow in FCI large address.
- Multi-collinear XC higher order derivatives.
- Smearing with predefined chemical potential.
- Fix frozen attribute for MP2 density matrices.

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