Implement option to calculate momentum using centroid speeds

[ole]

This calculation is currently part of the code balance_deep_and_shallow invoked by domain.tight_slope_limiters = True (see source:/anuga_core/source/anuga/shallow_water/shallow_water_domain.py).

However, if pulled out as an independent option, we can disable the gradient-limiter for momentum altogether and thus improve the overall speed of ANUGA when this option is switched on.

I understand that we may develop velocity limiters at a later point and use those to reconstruct momentum. However, this a step in that direction and will provide a further gain in computation speed.

[ole]

Done in changeset:5297

[ole]

This was really not a good idea to do in general. While it makes sense to use this approach to stabilise in very thin layers of water (as used in balance_deep_and_shallow) the method would make more sense as part of the first order scheme. I reopen the ticket to do this, while moving the centroid_velocity code back into the thin layers only.

[ole]

General use of centroid_velocities was removed again in changeset:5315 and only used for alpha<1 as before. The only difference now is that this functionality has its own keyword, use_centroid_velocities, to be used with tight_slope_limiters for example.