Specifically, refine the representation to allow a boundary condition
to apply to a set of interfaces rather than just a single face. This
support is realised by the introduction of new interface
flow_conditions_append_multi().
Update all consumers of struct FlowBoundaryConditions to accommodate
the one-to-many boundary condition interface mapping in the process.
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.
