Add output readback

[redpiramide]

This PR will introduce the following changes:

• Add a new Operation Operation::ReadWrite with RW in the asynUser text
• Add all variables with Operation::ReadWrite to both lists ads_read_vars and ads_write_vars
• Prevent readback caused by info(asyn:READBACK, "1") by allow to write only if ads is connected
• On write, a flag and the written value is stored in ADSVariable. Until the written value is read back, the written value will be returned.
• Update of the documentation and examples

The new Operation::ReadWrite should not interfere with existing configurations. I have successfully tested the readback with all scalar and array datatypes.

Closes #6

[exzombie]

I finally found some time to look at this. Thanks for posting it! I haven't tested it yet because I need to set up the PLC first. But at a glance, the PR looks nice. I always appreciate it when people take time to update the docs and examples together with the code 😄. There are two things that occurred to me while reading the code, addressed in separate comments.

[exzombie]

Looks good to me. Many thanks! Testing took a while because it exposed unrelated issues, some of them seem to be in asyn: it looks like mbboDirect records ignore asyn:READBACK. I will need to triage that further, but it doesn't prevent merging of your work.

I took the opportunity to make a small tweak and remove a bit of deprecated code. I will rebase and squash, hopefully tomorrow.