Add name-property to SequenceBaseBlock and implement access blocks of a Sequence by name

Often it makes definition of blocks to be added to an existing sequence easier if properties of a specific sequence-block can be accessed. To prevent the access via the private member Sequence._blocks it might make sense to add a member SequenceBaseBlock.name that is set on instantiation. Furthermore, a propertie Sequence.blocks could be implemented that returns a list of available block-names. A method Sequence.get_block(name) could than be used to acces the blocks.