2.0.0 - Major objects overhaul, fixes, + full market support

• Overhauled all objects in privex.steemengine.objects - converted to dataclasses using DictDataClass, with various dynamic properties allowing querying related data via SteemEngineToken.

  Every single object was refactored, so I'm not going to go into details on each class that was refactored.

• Added several new objects to privex.steemengine.objects - as dataclasses using DictDataClass.

• Added SteemEngineToken.native_coin and SteemEngineToken.native_token, to track the native market symbol of the network, e.g. SWAP.HIVE / STEEMP.

• Added new method SteemEngineToken.place_order which allows placing orders on the Steem/Hive Engine market, returning the SEPlacedOrder dictclass object, which allows easily referencing the Steem/Hive Engine transaction object, and the trades which fulfilled the order.

• Rebased exceptions in privex.steemengine.exceptions to use base class SteemEngineException

• Added new exceptions NoResults and NoSteemEngineInstance

• Added r_cache caching to various methods in SteemEngineToken for data which doesn't change often.

• Changed default indexes to [] (empty list) for query methods in SteemEngineToken, as a lot of indexes do not work on HiveEngine.

• Refactored the base functionality of order_history into query_order_history to simplify non-symbol queries in other methods

• Added new methods SteemEngineToken.find_fulfilled_sells, SteemEngineToken.find_fulfilled_buys, and SteemEngineToken.find_fulfilled which query using the new sellTxId / buyTxId keys

• Some improvements to the unit tests

• Various other small changes