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Add startup procedure to ChoCG

This adds logic to ChoCG setup to perform two elliptic solves as
described in

Christon et al, A hybrid incremental projection method for
thermal-hydraulics applications, Journal of Computational Physics,
vol. 317, p. 382-404, 206.

This involved adding code to:

* perform a projection of the initial velocity field to a div-free
  subspace (hence the first Poisson solve), followed by

* a second elliptic solve to compute the pressure that is consistent
  with its prescribed boundary conditions as well as the initial
  velocity field that is divergence-free. (For now, this second is a
  Laplace solve.)