Particle mkd only

[ldowen]

This PR adds:

• An assert to ensure dx[0] == dx[1] == dx[2]
• Changes spray droplet updates from to only doing a single move-kick-drift operation. Prior to this PR, the move-kick-drift operation updated the particle variables to dt/2 and the position to dt. Then a move-kick operation used the gas phase data at dt to update the particle variables to dt. Now, the move-kick-drift updates the particle position and variables to dt in one call. For particle CFL values greater than 0.5, this method is results in more consistent contributions to the gas phase. It also requires fewer redistribute calls.
• Removed unnecessary resetting of external source MultiFabs.