`integrate()` does not respect boundary conditions

[jbphyswx]

Similar to #97 , integrate() does not respect spline boundary conditions

Example:

using Dierckx
FT = Float64

x = FT[0,1];
y = FT[1,1];

spl = Dierckx.Spline1D(x, y, k=1; bc="extrapolate");

int_0_1 = Dierckx.integrate(spl, 0.0, 1.0);
int_1_2 = Dierckx.integrate(spl, 1.0, 2.0);

@info "integrals" int_0_1 int_1_2

@info "spline" spl(1) spl(2)

yields

julia> @info "integrals" int_0_1 int_1_2
┌ Info: integrals
│   int_0_1 = 1.0
└   int_1_2 = 0.0

julia> @info "spline" spl(1) spl(2)
┌ Info: spline
│   spl(1) = 1.0
└   spl(2) = 1.0

[jbphyswx]

It looks like the integral is done through a call to an outside library, for now I'm reconstructing the integral outside the knots manually, which is doable for "nearest" and "extrapolate". "error" of course just errors and "zero" of course is just 0.