Solve of isolated parallel networks

[tlmerbecks]

Where two or more hydraulically, thermally and chemically separate networks have been defined under one Network - either intentionally or "accidentally" by disabling connecting components (see #600) - the solve should ideally be performed separately. Because,

• if solved as one, the convergence of one depends on the convergence of the other
• the reduction in matrix size speeds up the calculations, although, due to overheads, not all of the potential gains may be realised.