C grid computational efficiency

[JFLemieux73]

We can list ideas of what we think could be done to reduce cpu time.

-in div_stress there are many calculations such as dxT(i,j)**2...can we store these as new arrays?
-if visc_method='avg_zeta', the halo update of the ice strength is probably not needed. Same idea if visc_method='avg_strength' I think the halo update of the viscosities is not needed...
-there are unnecessary calculations in strain_rates_U if visc_method='avg_zeta'.

[JFLemieux73]

One thing also to consider is that if you sum up the number of u an v that are calculated, this number is ~5% larger for the C grid (estimation from GLOBAL gx3 simulations). This is more pronounced (~10%) for the Northern hemisphere (more channels...e.g. in the CAA).

FYI @apcraig @eclare108213 @dabail10