ENH: change in the discretization of part of the (E)SI sensitivity terms

Part of the (E)SI shape sensitivities depends of grad(Ua) & nf computed
on the boundary. Up to now, the code was only computing the normal part
of grad(Ua), to avoid the potentially spurious tangential component
which is computed on the cell center and extrapolated to the boundary
faces. However, for some objectives that are strongly related to the
stresses (e.g. moment, stresses), including also the tangential part of
grad(Ua) is necessary for E-SI to replicate the outcome of FI.

Extensive testing on a number of objectives/cases showed
- No regression when including the tangential part
- Improved behaviour in some rare cases (moment, stresses)

Hence, the tangential part is now included by default. The previous code
behaviour can be replicated by setting the useSnGradInTranposeStresses
flag to true.