Specifically, replace the existing flowbc_t (that was densely
represented on each interface, including internal interface and
external no-flow interfaces) with a new structure given by
struct FlowBoundaryConditions
The semantics of this structure mirror those of "struct Wells" from
<opm/core/newwells.h>, but is currently mostly intended for simple,
incompressible flow purposes.
Update pressure solvers supporting boundary conditions to accommodate
the new boundary condition representation in the process.
