Rounding of Meter Values

[Pietfried]

OCPP Version

OCPP1.6

Describe the bug

ocpp's 1.6 start and stop transaction messages transmit the imported energy as integers. The current chargepoint implementation truncates the value from float to int32 in case of start_transaction, but does a rounding in case of stop_transaction. So in case no energy would be transfered with an initial value of 133.7, start_transaction would transfer 133 and stop_transaction would transfer 134.

Related zulip discussion: https://lfenergy.zulipchat.com/#narrow/stream/417676-EVerest.3A-Cloud-communication/topic/ocpp.20start.2Fstop_transaction

To Reproduce

Execute a charging session where no energy in transferred an check start and stop meter value.

Anything else?

A way to solve the issue described above is to add consistent rounding to start and stop meter values (e.g. use std::round for both start and stop meter values) that are transmitted as part of a StartTransaction.req and StopTransaction.req.

We can then make the rounding configurable in a next step.

[Pietfried]

Fixed with #646