Convenience functions

[jagot]

Some convenience functions that would be useful in different applications (names pending):

• issingles: tests if a CSF corresponds to a single excitation from an otherwise filled configuration. Useful for single-active-electron approximation calculations, which work well in excitation/ionization of closed-shell atoms (noble gases) and alkali metals (single valence electron).
• coupled_levels: given a set of CSFs and their levels (possible values of m_j), find the minimum set which is coupled from an initial state by an operator. E.g. the Cartesian z operator only couples Δm=0, which reduces the state space to a problem of 2d complexity.
• issingleslater: tests if a CSF corresponds to a single Slater determinant.
• to_slater: transforms a CSF to a linear combination of Slater determinants.