omci-lib-go enhancements for ONU SW upgrade

There are some smaller issues concerning the handling of SW upgrade that make it worth to improve to ease the usage of the omci-lib for some application. Things I detected so far in an order of priority:

1.) DownloadSectionRequest: It should be possible for the application to have control over the messageType.AR bit. The omci-lib should have no knowledge about the state of that bit. In current implementation it seems to me this bit is always set to 1 (did not find a way to modify it)

2.) DownloadSectionRequest::SectionData is defined to be (an array of) 29 bytes, but that should be 31 bytes. Moreover I would prefer this to be a byte slice which would eliminate the need of copy from used slices in the application (or other inconvenient pointer operations).