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Improved SCF algorithms for high-spin open-shell systems

For open-shell systems, RHF and RKS now use a two-step diagonalization process by default: Here the beta orbitals are found by a second diagonalization in the subspace of occupied alpha orbitals. This process usually leads to more stable convergence in difficult cases, compared to the standard diagonalization of a single open-shell Fock matrix (the latter behavior is recovered by {rhf,algo=0}). An additional section with suggestions for dealing with difficult cases has been added to be manual under ``The SCF program''. Additionally, various limitations of the SCF program have been lifted. In particular, {rhf; maxit,1} now always works, and just calculates a Fock matrix and energy from the input orbitals, without updating said orbitals. 2018-06-21