Many Rivers

[CyprienBosserelle]

Improve efficiency if may rivers are involved

I'm changing the structure of the model workflow to be efficient if a lot of rivers are in the model.
Currently kernels are launched in serial for each river. This is very inefficient when you have many rivers.
In an example using a crud model of the lower Waitaki River.

n rivers	Run times
1	6
10	7
100	14
675	71

This doesn't look too bad but things gets worse in other setting (may need a better example to make the point about the point of this)

[CyprienBosserelle]

using the same test:
675 river: 19s runtime!

[CyprienBosserelle]

Using the same setup as above but rerunning each scenarios. (Waitaki domain, single level (256m res). 3600 s run )

n rivers	Run times old code	Run times new code
1	5	5
10	5	6
100	14	7
675	73	9-10

Results between old and new code are exactly identical but the new code is 7 times faster with 675 rivers.

Tip

SUCCESS

[CyprienBosserelle]

Two Birds, one PR

Momentum conserving rain input

Also added a momentum preserving correction for u and v when adding rain input.