Add Dimension.label replacing global alias state

[philippjfr]

Addresses #1001 and supersedes #1075.

[philippjfr]

This is now passing without any major issues. This needs a lot more unit tests still and @jlstevens promised to write a short tutorial but we're both very happy with how this will work out.

[jbednar]

Looks good to me. Some examples of usage would be good.

[philippjfr]

Yes, those should be part of the tutorial. I'd actually like to suggest to broaden the scope of that tutorial to discuss Dimensions in general, with a significant section devoted to name and label but also covering the redim method and the difference between range and soft_range.

Another thing I've done in this PR is using strict dimension lookups in user facing APIs, which should result in much better tracebacks when referencing non-existent dimensions in many cases.

[jlstevens]

Thanks @philippjfr!

I'll shortly write up a notebook tutorial describing aliases, redim and the distinction betweenrange soft_range and add it to this PR.

[jlstevens]

I now have part of a Dimensions tutorial but I would like to work on #843 before continuing work on it. I am happy with this PR and would like to merge it now. Are you ok with me adding the Dimensions notebook to a different PR?

[philippjfr]

Sure, let's see how it goes. Better to get it in early to find issues.