Enabling the pure Neumann bc for all cap surfaces

[ju-liu]

The code can essentially enable full Neumann for cap surfaces. There are a few minor things to take into account.

1. The inflow file shall allow num_nbc to be zero;
2. The backflow stabilization term shall be turned off, at least for the `inflow' caps;

With the above two, one can enable the full Neumann cap bc by setting num_inlet to zero, and prepare all caps meshes with the name starting by outflow_vol_.

[ju-liu]

It can be better if we rename things with inflow / outflow by velocity / traction.

In doing so, it can be less confusing by setting all cap surfaces as traction bcs.