Add price feed rounds

[iFrostizz]

Closes #63

Add rounds to have historical rounds data like getRoundData and other missing methods.

• Make sure that all the relevant public functions at the AggregatorV3 level are included
• Check the equivalence of function signatures

Only the getRoundData has been added so that the PriceFeedImporter contract implements the AggregatorV3Interface. It is a good security practice for callers because it gives by default the answer as well as the update time of the answer which can be checked to judge staleness and fallback on other methods if this one is not satisfying enough. Methods such as latestTimestamp or latestAnswer are deprecated.
The method latestRound to get the latest roundID though may be useful (methods are at https://github.com/smartcontractkit/libocr/blob/ae747ca5b81236ffdbf1714318c652e923a5ff4d/contract/OffchainAggregator.sol#L721-L785) and in this case we may want to still implement those for legacy purposes ?

[iFrostizz]

This happened when formatting

[iFrostizz]

Events should be published monotonically increasingly. This is not true apparently on startup because the blocks catch-up operation is done asynchronously to the tip following one, thanks @cam-schultz !
We may want to modify this logic because of the catch-up so that it can accept any height, but not allow overwrites.

[iFrostizz]

An issue arising from that is that there might be some missing rounds although the last one is saved. Some smart contracts relying on this information to calculate i.e. the median price may get surprised by this behavior.
If we assume that any gap can only happen at the startup and that it would be filled quickly, we could save them in a "buffer" and only respond with the getRoundData method if none of the previous round is missing.