Add callback method as argument to dpm.add_entry

[beauremus]

@kjhazelwood: One of the few things I liked about the java DaqJob scheme.

[beauremus]

@kjhazelwood: Perhaps the dpm coudl group like callback method requests and map the callback to a Thread? Could this be a way to avoid having to use asyncio?

[kjhazelwood]

Like #4, it may be nice to have an arguments field as well so that the callmethod could receive these.