Index (usize) metric improvements.

[jetuk]

The current DynamicIndexValue is the usize equivalent of Metric. I think the former should brought more inline with the later setup. This would remove the nested enums in favour of a more "flat" approach. Tasks:

• Use ParameterReference instead of String
• Support index (usize) timeseries
• Support indes (usize) inter-network transfers

[jetuk]

Do this after #280 has been merged.

[jetuk]

The type should probably be u64 and not the platform specific usize.