Fix abs perm calculation

[JamesEMcClure]

Absolute permeability should include a factor porosity squared. Check that all LBPM routines are correct.

[yning2]

@JamesEMcClure Is there any reference that suggested using porosity squared for absolute permeability calculation. In my opinion, the average velocity across all fluid cells calculated in the code is the physical velocity and only multiplying porosity once is needed to convert the physical velocity into superficial velocity (i.e., Darcy velocity) for perm calculation. Welcome your thoughts on this. Thanks!