Reorganize datastore inheritance

[alexrudd2]

This solves 8 more mypy errors, including a real problem in ModbusSparseDataBlock.

The method default() (inherited from ModbusBaseDataBlock) does not make any sense for a sparse datastore, where self.values is a dictionary. If called, it will break the implementation.
==> Move it to ModbusSequentialDataBlock.

Also, reset() is defined in ModbusBaseDataBlock but overridden by ModbusSparseDataBlock.
==> Move it also to ModbusSequentialDataBlock.

[janiversen]

LGTM, thanks.

However it scares me a bit, that no tests were affected .... we really need a lot more testing.