Add ability to add a chain of transfers

[crteensy]

When multiple tranfers must be executed in a known order, therre should be a possibility to add them as a block (including possible loops, if desired). This means that data that is private to the transfer must be manipulated before the transfer is registered with the DMASPI object.

• add a class that can create transfer chains
• modify registerTransfer() to accept a chained transfers or add a registerTransferChain() method