assembleDynamic(): Fix out-of-bounds indexing in children

In the interest of reducing the number of allocations/deallocations,
the 'rhs' and 'gflux' arrays are allocated once (of maximum size,
'max_ncf_') at the start of assembleDynamic().  However, method
buildCellContrib() implicitly assumes that 'rhs' and 'gflux' are
sized according to the number of interfaces on a given cell.  This
discrepancy leads to out-of-bounds indexing unless all cells have
the same number of interfaces.

Explicitly resize the 'rhs' and 'gflux' arrays for each cell in
assembleDynamic() under the assumption that no additional memory
operations occur during vector<>::resize() if the .capacity() is
large enough.  This way, buildCellContrib() will only inspect the
appropriate number of array elements, i.e., those that correspond to
the current number of cell interfaces.

This bug was present for a *long* time.

Superb detective work by: @akva2
Suggested solution by: @atgeirr
computePressureAndFluxes(): Fix out-of-bounds indexing

In the interest of reducing the number of allocations/deallocations,
the 'pi' and 'gflux' arrays are allocated once (of maximum size,
'max_ncf_') at the start of computePressureAndFluxes().  However,
those variables are subsequently used in way that assumes that
variables are sized according to the number of interfaces on a given
cell.  This discrepancy leads to (semantic if not physical)
out-of-bounds indexing unless all cells have the same number of
interfaces.

Explicitly resize the 'pi' and 'gflux' arrays for each cell in
computePressureAndFluxes() under the assumption that no additional
memory operations occur during vector<>::resize() if the .capacity()
is large enough.  This way, the std::transform() calls will only
reference the appropriate array elements, i.e., those that pertain
to the current number of cell interfaces.

This bug was present for a *long* time.

Detective work and suggested solution by: @atgeirr
Merge pull request #92 from bska/fix-out-of-bounds-indexing

Fix out-of-bounds indexing in IncompFlowSolverHybrid