Improve field parsing

[bobmyhill]

ASPECT has "compositional" fields not only for compositions, but also other properties like grain size and stress (and soon, a lot of other fields with the updates planned by @MFraters). Some material models require certain fields in specific (but different) orders, and different rheologies don't play nicely with the others. With the increasing modularisation of the material models, I think this may become a significant stumbling block.

Some potential improvements:

• [ ] Fix Rheology::Elasticity so that it doesn't require material parameters for the stress fields.
• [ ] Make a new field label parameter, to automatically identify the type of field to which each name corresponds
• [ ] Make masks for each type of field
• [ ] Update the material models so that they don't care about the order of different fields.
• [ ] Change "Compositional fields" to "Fields"? That would be a big change.

Some of these changes are likely to break backwards compatibility, so I'm opening the issue for discussion.

[tjhei]

Agreed. I think we should have a conversation about this.

Related #4182

[naliboff]

Agree this needs to be discussed and that many of the proposed changes are needed, regardless of whether backwards compatibility is broken. #3927 is also related.