Fix methods that referred to names that no longer exist

[jishnub]

A JET pass over the package surfaced a set of methods that could not have been called since Julia 1.0 — they refer to variables or functions that are not defined, so they throw UndefVarError/MethodError on the first line that touches them. Most are typos and are repaired; two methods are removed.

Repaired

Location	Problem	Fix
LinearAlgebra/lyap.jl:74	schurfact and AC[:left]/AC[:S] — the pre-1.0 Schur API	schur, .Q/.Z/.S/.T
Domains/Point.jl:2	new used outside a struct body	Point{T}(T(NaN)), matching convert(::Type{Point{T}}, ::AnyDomain) on the next line
Domains/ProductDomain.jl:34	pushappendpts!(ret, SVector(x), pts) with x undefined	pushappendpts!(ret, (), pts); the accumulator is a tuple, as in checkpoints above it
PDE/KroneckerOperator.jl:259	sp.c — no sp, and ConstantTimesOperator's field is λ	S.λ
Operators/SubOperator.jl:337,349	"Cannot convert $V to a $TYP" — inside @eval, $TYP in a string literal is a runtime reference to a global TYP	$($TYP)
Operators/almostbanded/LowRankOperator.jl:30	eltype(eltype(v)); the parameter is V	eltype(eltype(V))
Spaces/Spaces.jl:110	BB used on the right-hand side of its own definition	bind ds = domainspace(A[end]) first (all of A shares a domain space after promotedomainspace)
Spaces/Spaces.jl:113	VectorSpace(...) — VectorSpace is a type alias, not a constructor	build the range space with Space(::AbstractArray), as LowRankOperator(_, VectorSpace) already does
Caching/blockbanded.jl:47	diagblockshift(flatten((v, w), b)) — flatten is undefined and b is inside the wrong call	diagblockshift(Vcat(v, w), b), matching the neighbouring branches
Spaces/ConstantSpace.jl:81-82	zero(ConstantSpace()) — there is no zero(::ConstantSpace)	zeros(ConstantSpace())
Spaces/ConstantSpace.jl:106,108	bare promote_op inside Base.promote_op definitions	Base.promote_op
Spaces/ConstantSpace.jl:9	_first_or_zero(f) with f::Fun; the method takes an AbstractVector	f[1], which already handles the empty case
specialfunctions.jl:87	minabs, removed in Julia 1.0	minimum(abs, f)

Removed

• isapprox_atol(::Number, ::Number, ::Real) and isapprox_atol(::AbstractArray, ::AbstractArray, ::Real) in LinearAlgebra/helper.jl. Both defaults are dead: Base.rtoldefault(x, y) (two-argument form, removed in 0.7) and vecnorm (removed in 1.0). They were a fast path around the generic isapprox_atol(a, b, atol; kwds...) on the line above, which handles both cases correctly, so removing them leaves every working call unchanged.
• SubSpace(sp::SubSpace, kr) in Spaces/SubSpace.jl. It calls to_indexes (removed from Base) and a three-argument reindex that no longer exists. Nesting now goes through the generic SubSpace(sp::Space, kr).

Verified

Each repaired method was exercised in a REPL:

• isapprox_atol on Number/AbstractArray/SVector
• lyap(A,B,C,D,E) — residual ‖A X Bᵀ + C X Dᵀ − E‖/‖E‖ ≈ 6e-16 on a random system
• Point{Float64}(AnyDomain()), zeros(AnyDomain()), zero(UnsetSpace())
• points((-1.0..1.0)^2, (3,3)) → 9 points
• Base.promote_op(*, typeof(Fun(1.0)), typeof(Fun(x->x, Chebyshev())))
• Fun(SequenceSpace(), [3.0,4.0])[CartesianIndex()]
• isfinite(Fun(x->x^2, Chebyshev()))
• transpose(2.0*Derivative(Chebyshev()))
• the Matrix(::SubOperator) error message
• LowRankOperator(U, V) with mixed element types
• convert(Operator, Operator[Evaluation(Chebyshev(),-1), Derivative(Chebyshev())]) — this reaches LowRankPertOperator, and now returns a working operator
• diagblockshift(Vcat([3,1,1], Ones{Int}(∞)), Ones{Int}(∞))

test/runtests.jl gains a testset covering the cases that need only ApproxFunBase (11 assertions). Pkg.test("ApproxFunBase") passes, and so does downstream Pkg.test("ApproxFun") (1048 pass, 2 broken, 0 fail).

Still broken, flagged rather than fixed

Base.real/Base.imag for a ProductFun over a complex basis (Multivariate/ProductFun.jl:414-416) referred to an undefined u; that is a typo for f and is corrected here. The methods still do not work, though: they build ProductFun(real(f.coefficients), space(f)), which pairs real coefficient Funs (CosSpace ⊕ SinSpace) with the original complex space (Laurent ⊗ Laurent) and throws an InexactError. Making them work needs more than a rename, so they are left as a known gap — happy to remove them instead if you would rather not keep a non-functional method.

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