Implement virtual node penalty costs

[jetuk]

This was output from the last developer's meeting and subsequent discussion. It was felt this would simplify applying licence costs to nodes using those licences.

• Virtual storage nodes (and some others) have their own penalty costs applied to attached nodes.
• have an option for a node's cost to be the min (or max, sum, etc) of those affecting it.

Options for defining how a node's costs are aggregated:

{
    "name":  "source1",
    "type":  "input",
    "max_flow": 10.0,
    "cost": {
         "type":  "virtual"
         "agg":  "min"
    }
}

{
    "name":  "source1",
    "type":  "input",
    "max_flow": 10.0,
    "cost_agg": "min",
    "cost": 10.0
}

{
    "name":  "source1",
    "type":  "input",
    "max_flow": 10.0,
    "cost": {
        "value": 10.0,
        "agg": "min",
    }
}

[jetuk]

Initial implementation merged in #71

Outstanding actions:

• Some good documentation especially as its a pretty significant change from v1.
• The ability to specify separate agg funcs to apply to the virtual costs and to the aggregated virtual cost and the local cost could be useful.