Engineering domain: Structure Design

## Task Description

This is a mulitple design load problem. 
Design domain is a 2D rectangular region in cartesian coordinate, with vertices at (0,0), (6,0), (6,1), (0,1). 
Vertically downward point loads are applied at (2,1) & (4,1). Pin support is at (0,0) and roller support is at (6,0). 
Lx = 6 and Ly = 1.
The filter radius is R = 0.03*max(Lx,Ly).
The filter exponent is 3.


---

### Task

Your task is to:
- Obtain a topology optimized design that has minimum structural compliance and volume fraction not exceeding 0.25.
- Given the optimized design, output the corresponding minimum structural compliance, named C_y_hat, and also its volume fraction, named vf, in numerical numbers. Note that vf is in the range of [0,1].