feat: implement scale_factor and add_offset getters for both readers

[utkarshkukreti]

Adds scale_factor and add_offset getters for both OmFileReader and OmFileReaderAsync.

Both of these follow a similar approach as the existing compression_name getter - raising an error when called on non-array values.

---

I mostly just followed how compression_name was implemented. Please let me know if I missed anything.

[terraputix]

Good work! Thank you for the contribution :)

[utkarshkukreti]

@terraputix thanks! Will this be in the 1.2.0 release? Any estimate on when it will be out?

[terraputix]

@utkarshkukreti yes it will be included in the 1.2.0 release (compare #115).
I'd like to get some other things into that release, but I think I'll be able to release this at some point next week.

If you have all the tooling to build this project yourself, you can always fall-back to a direct git dependency like "omfiles @ git+https://github.com/open-meteo/python-omfiles.git#a3628ddd27cd18c14ba763580cfe8533bde36fbf", in your pyproject.toml dependency list.

[utkarshkukreti]

Thanks, that worked fine on Mac. On Linux I had to install clang to make it work. Looking forward to the release.