Provide a dedicated subroutine to evaluate the integrated value of a field

[mathrack]

Several cases estimate the integrated value of a field on

• a face, for instance the inlet or the outlet
• a volume, for instance to estimate the total flow rate in the domain

In case of stretching, the integration is often approximate :

https://github.com/xcompact3d/Incompact3d/blob/55652680be067db9336161b2a9c873744c637811/src/Case-PTBL.f90#L1350

It would be best to use the stretching (ppy) to integrate exactly fields :

https://github.com/xcompact3d/Incompact3d/blob/55652680be067db9336161b2a9c873744c637811/src/Case-Channel.f90#L240

It might be best to provide a dedicated module with subroutines to integrate fields on faces and volumes to avoid code duplication.