title: Gaussian Density Pulse Test Case

This a validation testcase for Acoustic 2D. We consider here a gaussian density pulse, which 
travels due to advection from one side of the domain to the other, while all boundaries are periodic.
Hence no boundary conditions are defined. 
**Features used**

1. Projection: fpt

2. Polynomial representation: Q

3. Filtering: -- 

4. Timestepping: explicitRungeKutta, 4 steps 

5. Boundary conditions: -- 

6. Others: 
   - Derived quantity: Density

