Add Simpson's method on irregular grid + tests

[IlianPihlajamaa]

Added the methods

integrate(x::AbstractVector, y::AbstractVector, ::Simpson)
integrate(x::AbstractVector, y::AbstractVector, ::SimpsonFast)

which work on irregular grids.

Also added a testset to test the new methods (together with Trapezoidal/Trapezoidal Fast)

Example:

julia> x = LinRange(0, sqrt(π/2), 10).^2
10-element Vector{Float64}:
 0.0
 0.01939254724438143
 0.07757018897752573
 0.17453292519943292
 0.3102807559101029
 0.484813681109536
 0.6981317007977317
 0.9502348149746904
 1.2411230236404116
 1.5707963267948963

julia> y = sin.(x)
10-element Vector{Float64}:
 0.0
 0.019391331771824363
 0.0774924206719309
 0.1736481776669303
 0.305325997695113
 0.4660435197025388
 0.6427876096865393
 0.8135520702629675
 0.9461481568757503
 1.0

julia> integrate(x, y, Simpson())
0.9998087840617803 #analytic result is 1