Incorrect inverse in some cases

[oschulz]

With

using MonotonicSplines

w = [0.0, 0.15, 0.20, 0.45, 0.65, 0.80, 1.0]
h = [0.0, 0.10, 0.25, 0.40, 0.55, 0.70, 1.0]
d = [0.5, 1.3, 0.8, 0.9, 0.7, 1.2, 1.1]
X = collect((0:0.1:1)')
n = length(X)
M = fill(1.0, 1, 1, n)
f = RQSpline(w .* M, h .* M, d .* M)
inv_f = InvRQSpline(w .* M, h .* M, d .* M)

Y = f(X)
X2 = inv_f(Y)

we currently get

julia> X ≈ X2
false

julia> X
1×11 Matrix{Float64}:
 0.0  0.1  0.2  0.3  0.4  0.5  0.6  0.7  0.8  0.9  1.0

julia> X2
1×11 Matrix{Float64}:
 0.0  0.1  0.2  0.3  0.4  0.5  0.6  0.7  0.8  0.85283  1.0

but when adding a spline node at (1.1, 1.1)

w = [0.0, 0.15, 0.20, 0.45, 0.65, 0.80, 1.0, 1.1]
h = [0.0, 0.10, 0.25, 0.40, 0.55, 0.70, 1.0, 1.1]
d = [0.5, 1.3, 0.8, 0.9, 0.7, 1.2, 1.1, 0.9]
X = collect((0:0.1:1.1)')
n = length(X)
M = fill(1.0, 1, 1, n)
f = RQSpline(w .* M, h .* M, d .* M)
inv_f = InvRQSpline(w .* M, h .* M, d .* M)

all is correct:

julia> X ≈ X2
true