FEAT: Implement Viscosity

[aaronjridley]

Is your feature request related to a problem? Please describe.
Need to implement viscosity in the momentum equation.

Describe the solution you'd like
Create a file called neutral_momentum_viscosity.cpp and implement in there. In GITM there is the viscosity of the horizontal winds in the vertical direction, but we should implement this on the vertical winds in the horizontal direction. I am not sure, but we should be able to use the conduction solver for the viscosity in the vertical direction.

Describe alternatives you've considered
Should look at GITM and see how it is implemented there. There is no horizontal viscosity, but should be able to implement this with "simple" gradients.

Additional context
Look at GITM for the vertical. Need to implement gradients in the horizontal direction.