Handle EnrichedElements better

[mscroggs]

• Allow the user to enrich spaces by either preserving the functionals, or the basis functions (Add enriched element option to preserve the functions rather than the dofs #580)
• Allow non-interpolatory enrichments (eg continuous P1 enriched with DP0)
• Check that discontinuous is given the right value when enriching a combination of continuous and discontinuous spaces

[mscroggs]

Having enriched elements that preserve partition of unity has been requested: https://fenicsproject.discourse.group/t/question-about-scaling-of-bubble-enriched-basis/13729/2?u=dokken

[garth-wells]

Having enriched elements that preserve partition of unity has been requested: https://fenicsproject.discourse.group/t/question-about-scaling-of-bubble-enriched-basis/13729/2?u=dokken

Is that even possible in general?

[mscroggs]

Having enriched elements that preserve partition of unity has been requested: https://fenicsproject.discourse.group/t/question-about-scaling-of-bubble-enriched-basis/13729/2?u=dokken

Is that even possible in general?

I don't think so. Thinking about this I think what we're after there is an enriched element that preserves the functionals which this issue already asks for