Adding VelocityBoxes to the setup and their respective utils/tests

[jaime-almeida]

Added a new structure to BoundaryConditions.jl, VelocityBox, to allow for the use of these boundary conditions. They follow the definitions on the LaMEM source and ask for the same variables to be set. I've also added the utility functions for this structure, add_vbox!(), rm_vbox!() and rm_last_vbox!() to the Utils.jl file.

vbox = VelocityBox(cenX=0, cenY=0, cenZ=0,
                        widthX=1, widthY=1, widthZ=1,
                        vx=1)
add_vbox!(model, vbox)

I also edited the print function in BoundaryConditions.jl to allow for this structure to be written into the input file following the LaMEM structure:

<VelBoxStart>
      cenX      =  0  # X-coordinate of center of box
      cenY      =  0  # Y-coordinate of center of box
      cenZ      =   0 # Z-coordinate of center of box
      widthX   =  0  # Width of box in x-direction
      widthY   =   0 # Width of box in y-direction
      widthZ   =   0 # Width of box in z-direction
      vx           =   1 # Vx velocity of box 
      advect   =   0  # box advection flag
<VelBoxEnd>

I also added a test to ~test_julia_setups.jl which verifies that the velocity box was correctly placed.