xyst

WIP towards startup procedure in ChoCG

Involves:

* Add a way to configure Dirichlet and Neumann BCs for velocity and
  pressure: pressure = { ... bc_dir = { ... }, bc_sym = { ... } .. }

* Diagnostics aggregation now carries the number of scalar components

* By default the number of prognostic variables in ChoCG is 3 (=n-2),
  derived from n configured upstream, see InciterConfig.cpp.

* Compute gradient of scalar variable after initial elliptic solve with
  the rhs as the divergence of the initial conditions for the velocity
  field.  Both divergence and gradient need communication.

* Revamp setup of BCs in ChoCG to reuse code between momentum and
  pressure BCs.

* Minor updates to Poisson regression tests control files