Added support for DPT 24.* and DPT 28.*.

[ab-tools]

No description provided.

[ab-tools]

Another question, noticed while working on this:
What is the reason that signed8FromPayload is used for 14 byte strings here instead of unsigned8FromPayload?
https://github.com/OpenKNX/knx/blob/v1/src/knx/dptconvert.cpp#L524

This is even more strange as unsigned8ToPayload is used for the opposite conversion direction:
https://github.com/OpenKNX/knx/blob/v1/src/knx/dptconvert.cpp#L1196

Is there a specific reason for this or a bug that should be fixed?

[Ing-Dom]

please change your PR to merge into v1dev instead of v1

[ab-tools]

Sure, done.

[Ing-Dom]

thx. and please merge the latest v1dev to your feature branch then the checks should all be ok

[ab-tools]

Done as well.

[ab-tools]

Will create new PR, when ready - as discussed in Slack.