grade selection syntax

When getting the elements of a `MultiVector`, you can use parentheses to select the grade, i.e.

```
A = 1 + 2v1 + 3v12 + 4v123 # 1 + 2v₁ + 3v₁₂ + 4v₁₂₃
typeof(A) # MultiVector{⟨1,1,1⟩, Int64, 8}
sizeof(A) # 64
A(0) # 1v
```

However, if the type if a `Chain`, then the parentheses act as an indexing operation, and only into the elements that are non-zero in the `Chain`. Attempting to access the scalar part of a `Chain` via `(0) returns an index error.

```
B = 3v12 + 4v23 # B = 3v₁₂ + 0v₁₃ + 4v₂₃
typeof(B) # Chain{⟨1,1,1⟩, 2, Int64, 3}
sizeof(B) # 24
B(0) # BoundsError: attempt to access 3-element Vector{UInt64} at index [0]

```

There is a work-around for this, it is to wrap the `Chain` in a `MultiVector`, but a `MultiVector` is (depending on the basis) many times the size of a `Chain`. It seems to me that this behaviour of `Chain` is useful for the internal implementation, but at the user level, the difference between a `Chain` and a `MultiVector` should be abstracted away.

Start by reproducing the documented calls on both MultiVector and Chain, especially B(0), and compare their grade-selection and indexing behavior. Determine the intended user-facing behavior for Chain without expanding it to a MultiVector, then verify that scalar and nonzero-grade access work consistently.

Written by the indexing model from the issue text.