on CLang 3.4 svn this produced
```
/home/erne/src/opm-core/opm/core/pressure/fsh.h:20:9: warning: 'OPM_FSH_HEADER_INCLUDED' is used as a
header guard here, followed by #define of a different macro [-Wheader-guard]
```
which is correct...
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.
