allow user to control the use of PV put completion

[hinxx]

Possible solution for #2.

[darcato]

Why not having an optional argument on the put method, for example wait_complete=True?

[hinxx]

You're right, having an optional argument on the put itself seems more straightforward. I'll rework the PR and try to add doc change, too.

I was also thinking about the putComplete() matter. If the interpretation of it is strictly meant as an indication of async completion, then it would make sense that fsmIO._putComplete remains False in cases when wait_complete=False is used and the trigger with 'putcomp' is never issued in such case. If the PV put would fail the epicsIO.put() reports an error already.

What do you think?

[darcato]

I know that I proposed the name wait_complete but given the additional context probably it's better to stick with use_complete (the same as the underlying code) or use_callbacks, since this flag only enables/disables the put callbacks but it never waits for something.

[hinxx]

Re-based to current master code, too.