Qiskit / qiskit-fermions / 34127222595

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refactor(linalg)!: remove the FCI kernel in favour of ffsim (#326)

Simulation is not this package's concern. Feedback on the ffsim boundary
reported that shipping a full configuration interaction kernel puts
qiskit-fermions in the simulation business alongside ffsim and PySCF, when
its scope is fermionic mappers and the mapper-agnostic circuit and its
transpilation. The kernel was the largest single piece of code serving a
capability that ffsim already owns, so it goes.

`ffsim.SupportsLinearOperator` is now satisfied by converting a `FermionOperator`
into an `ffsim.FermionOperator` and delegating to `ffsim.linear_operator`.
The conversion deliberately bypasses the `SupportsFermionOperator` protocol:
despite the shared method name, `_fermion_operator_` returns *this* package's
type rather than ffsim's unrelated one, which is the incompatibility the
previous commit documented. It reads the flat term buffers instead of
iterating term objects, so a Hamiltonian's worth of terms does not pass
through intermediate Python lists, and it preserves the order of the ladder
operators within a term, since reordering them flips the sign of a cross-spin
term.

A `_trace_` method is added alongside it, satisfying `ffsim.SupportsTrace`. The
trace of an operator on a fixed sector preconditions `expm_multiply`, and a
SciPy `LinearOperator` has nowhere to carry it, so the evolution path asks
for it separately. Note that `ffsim.trace` rejects an integer `nelec` where
`ffsim.linear_operator` accepts one and reads it as an empty beta sector, so
the pair is spelled out to keep both spellings of a spinless sector working.

This costs simulation on Windows, which ffsim cannot support through PySCF,
and forecloses a C API for it, since the only path now runs through CPython.
Both are accepted: the alternative is maintaining a second simulation backend
to serve a platform ffsim intends to reach. Everything that does not simulate
is unaffected on every platform, and the release ... (continued)

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