Deprecate preliminary back extension support

[Peter9192]

In #123 support is added for the "normal" back extension. That means that the preliminary back extension will be less relevant. Since the preliminary back extension adds substantial complexity to this tool, we might want to think about deprecating it. Users relying on the back extension for the reproducibility of previous results could still use an older version of era5cli.

[malininae]

@Peter9192 Although I agree it's less relevant now, but there are still nine years that one wants to download through the back extension (1950-1958). I would suggest keeping it for a bit.

[BSchilperoort]

@malininae with the changes in #146, I assume the back extension can now be fully deprecated?

[malininae]

@BSchilperoort I suspect so. The only case speaking against it, if there is someone who wants to compare preliminary and final version. I don't know how much support the package has, but maybe say, it will be deprecated in the next release?

[BSchilperoort]

That could work. I'll make the usage of --prelimbe raise a deprecation warning, and remove it in a later release.