xyst

Fix conjugate gradients init procedure

- Previously the init procedure randomly led to a quiescence because of
  the reuse of m_nr for communicating both the rhs aggregating the BCs
  and the communication of the residual which followed. The vector and its
  communication buffer (m_r, m_rc) are still reused for these two steps,
  but now using an independent counter (m_na and m_nr) ensures that these
  two steps do not interfere with each other due to asynchronous task
  ordering, especially in parallel and overdecomposed.

- Comment out velocity initialization in ChoCG, not yet used and would
  trigger error due to incomplete code.

- The initial pressure solve is now invoked only after the boundary
  normals have been communicated. This will be useful for applying
  Neumann BCs on the initial pressure solve.

- Remove ignorebc arg from ConjugateGradients::init(), no need for that
  so far.