[multi_frame_4] incorrect input dimensions

[pressureless]

Create by @alecjacobson :

The vectors e₁ and e₂ are 2D. I propose changing this to:

Ω = [`e₁` `e₂`][`k₁`   0
                           0    `k₂`] [`e₁`ᵀ
                                          `e₂`ᵀ]

where
`k₁` ∈ ℝ  
`k₂` ∈ ℝ 
`e₁` ∈ ℝ²
`e₂` ∈ ℝ²

If we fix pressureless/linear_algebra#71 then we could write this even more tidily as:

Ω = [e₁ e₂][k₁   0
                  0    k₂] [e₁ᵀ
                               e₂ᵀ]

where
k₁ ∈ ℝ  
k₂ ∈ ℝ 
e₁ ∈ ℝ²
e₂ ∈ ℝ²