Read existing network and schedule from file

[olwijn]

For large cases (many wells, long history) the construction of a FlowNet case from scratch might take quite some time. For repeated experiments in which only history match settings are varied, significant speed-up could be obtained by re-using an existing architecture and schedule. These could be read from stored pickled files generated during a previous experiment. This functionality is already used when running FlowNet in prediction mode and could be made available also for experiments in history match mode.