Add custom basis

[ohno]

Please add a basis function that holds the function like the custom term of hamiltonian:

TwoBody.jl/src/Hamiltonian.jl

Lines 77 to 79 in c67ec58

	Base.@kwdef struct Custom <: PotentialTerm
	f::Function
	end

The function element(o::Custom, SGB1::SimpleGaussianBasis, SGB2::SimpleGaussianBasis) can be reused.

TwoBody.jl/src/Rayleigh-Ritz.jl

Lines 432 to 441 in c67ec58

	function element(o::Custom, SGB1::SimpleGaussianBasis, SGB2::SimpleGaussianBasis)
	value, _ = QuadGK.quadgk(
	r -> 4π * r^2 * φ(SGB1, r) * V(o, r) * φ(SGB2, r),
	0.0,
	Inf,
	rtol=1e-10,
	atol=1e-12,
	)
	return value
	end

One suggested name is CustomBasis.