Use @property decorators

[jorgensd]

I think we should start using property decorators, as it would make alot of the code more readable.

• Make methods into properties in AbstractCell #169

[wence-]

A number of the zero-argument property functions currently do some computation to cache expensive results, so it might be worthwhile considering functools.cached_property too.

[jorgensd]

I'm fine with using functools.cached_property as well. I figured a proposal such as this would break quite a lot of code in both DOLFINx and Firedrake, but as long as you, @dham , @garth-wells etc. are happy with this being introduced, Im happy to give it a go over a weekend.

[jhale]

It would be good to create a release/tag with a last working version before proceeding with large changes.

[dham]

I'm in favour of backward-incompatible API improvements in UFL. However I'd rather they came in a limited number of batches, rather than a constant dribble. Users should understand us changing our API for the better, but their appetite for repeatedly changing their code will be limited.

[jorgensd]

It would be good to create a release/tag with a last working version before proceeding with large changes.

The latest version working with DOLFIN (bitbucket) is 1de7f12 (or maybe 7a4303f)
The changes introduced in #77 breaks old ffc as noted in #77 (comment)