Computation of device usage time

[SergioMD15]

We need to compute the total time a device has been used. This value is dynamically computed taking advantage of the time of disk usage from their components.

This attribute diskUsage exists in FunctionProofs.sol, but needs to be added as well in DataWipeProofs.sol.

[SergioMD15]

Doubts:

• The value of the attribute diskUsage in a new proof shows the total lifetime of that disk or instead the time that has passed since the last proof?
  • Shows the total lifetime of a device.
• How do we know if a disk is new in a device when the proof is generated?
  • No need to worry about this now. Assume it is not changed.
• Should the timestamp be a GenericProof attribute?
  • Yes, it has been already added as an attribute taking advantage of block.timestamp.

Changelog

The deviceUsage feature won't be implemented in the end. Instead, what we will do is to create a new parent proof MetricProofs with DataWipeProofs and FunctionProofsas children. This new proof will hold values related to metrics that can be used outside of the blockchain.