Precompute indices in cuda-ico

[Stagno]

Instead of recomputing the "final" index to access a field each time (for each field), like:

... u_vert(kIter * NumVertices + nbhIdx) * dual_normal_x[kIter * NumEdges * E_C_V_SIZE + nbhIter * NumEdges + pidx]) + (v_vert[kIter * NumVertices + nbhIdx] ...

we should store, at the beginning of the relevant scope, such index, so that

• we help nvcc with common subexpression elimination
• can assert that the index is not out of bounds

const int sparseIdx = kIter * NumEdges * E_C_V_SIZE + nbhIter * NumEdges + pidx;
assert(sparseIdx < NumEdges * E_C_V_SIZE * kSize);