Add the option to inject constant flow into the vasculature

[nico-canta]

Now we have two options:

• we inject flow with constant velocity
• we inject flow with a sinusoidal velocity

However, the flow is never constant if one of the entry nodes is associated with an endfoot. The flow is Q = A v and the area A changes stochastically over time.

https://github.com/BlueBrain/AstroVascPy/blob/2dd34a4a80950dbe8efb4358e4349372057c29aa/astrovascpy/bloodflow.py#L591

We should introduce the option to inject constant flow into the vasculature. In this case we should ignore the endfoot connected to the entry nodes.