In the examples we can find the aneurysm example where is considered integral continuity. Although in the documentation nor in the code there is no further explanation of the idea behind that. In the code we can find

```
# Integral Continuity 1
integral_continuity = IntegralBoundaryConstraint(
nodes=nodes,
geometry=outlet_mesh,
outvar={"normal_dot_vel": 2.540},
batch_size=1,
integral_batch_size=cfg.batch_size.integral_continuity,
lambda_weighting={"normal_dot_vel": 0.1},
)
domain.add_constraint(integral_continuity, "integral_continuity_1")
# Integral Continuity 2
integral_continuity = IntegralBoundaryConstraint(
nodes=nodes,
geometry=integral_mesh,
outvar={"normal_dot_vel": -2.540},
batch_size=1,
integral_batch_size=cfg.batch_size.integral_continuity,
lambda_weighting={"normal_dot_vel": 0.1},
)
domain.add_constraint(integral_continuity, "integral_continuity_1")
```

It would be helpful to understand why it is setup in such specific way, using two integral continuity and why are there opposite signs of `normal_dot_vel`

(I assume that one is for inflow and the other is for outflow). As well further explanation of how to physically interpret the normal_dot_vel and how to handle it properly would be appreciated.