Add startRound code.

[sumasamurai]

This code is responsible for starting a new prediction round and recording its details in the rounds data structure. The StartRound event can be used to notify external parties or interfaces about the beginning of a new round.

event StartRound(uint256 indexed epoch);

function _startRound(uint256 epoch) internal {
    Round storage round = rounds[epoch];
    round.startTimestamp = block.timestamp;
    round.closeTimestamp = block.timestamp + 5 minutes;
    round.epoch = epoch;
    round.totalAmount = 0;

    emit StartRound(epoch);
}

[sumasamurai]

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