New version: BasisFunctions v0.7.0

[JuliaRegistrator]

• Registering package: BasisFunctions
• Repository: https://github.com/JuliaApproximation/BasisFunctions.jl
• Created by: @daanhb
• Version: v0.7.0
• Commit: c1cfd463b35c4b1f58665477d2813c64c721211a
• Reviewed by: @daanhb
• Reference: JuliaApproximation/BasisFunctions.jl@c1cfd46#commitcomment-149242608
• Description: A collection of methods for manipulating various well-known types of basis functions and recombining them into more general dictionaries
• Release notes:

## Breaking changes

- an expansion (dictionary+coefficients) no longer acts as the vector of coefficients. This means it cannot be indexed (as in `e[2]` returning the second coefficient) and cannot be iterated over
- there are some interface cleanups. The `norm` and `moment` functions with a combination of dictionary and coefficient as arguments are renamed to `dict_norm` and `dict_moment` instead. One can still invoke `norm` on a basisfunction, e.g., `norm(dict[3])`

## Features
- computing with expansions has improved. For example, computing the diagonal of the Christoffel-Darboux kernel of a polynomial basis can be done as follows:
```julia
julia> basis = orthogonalize(Legendre(3))
DiagonalOperator(v) * Legendre(3)

v = 3-element Vector{Float64}:
 0.707107
 1.22474
 1.58114

julia> k = sum(phi^2 for phi in basis)
Expansion(Legendre(5), v)

v = 5-element Vector{Float64}:
 1.5
 8.28036e-16
 1.71429
 1.14325e-15
 1.28571
```

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