Unify refine interface

[schnellerhase]

This PR contains several changes to the refinement routines to facilitate a neater, simpler and combined API to the user.

1. The interface of the plaza routines now uses std::optional's as input arguments and thus allows for a single code path. (Previously the cases of uniform and adaptive refinement were treated independently)
2. The plaza refinement options was centralized to the (more general) refinement namespace and now is also used for configuring the interval refinement.
3. The return values of parent facets and cells are now std::optional as well, to no longer need to rely on an ambiguous empty array check.
4. The python export and interface with the new optional arguments is greatly simplified, no longer do we need to export multiple versions of the refinement routines, and no more dispatching in the python interface is necessary.
5. Refinement testing has been extended and simplified in some cases.

[schnellerhase]

• Add functionality of retrieving the parent facet/vertex indices with the interval refinement
• Move refinement::plaza:Option for controlling what gets computed to central location
• Make ghost mode option of plaza
• Make interval refinement available through refine()
• python exports simplifications, one function one export - optional arguments should be treated implicitly by python caller
• Should probably return a shared_ptr to the new mesh, for consistent interface usage with the external API

[jhale]

"Should probably return a shared_ptr to the new mesh, for consistent interface usage with the external API" - This isn't necessary, as the caller takes ownership.

Could you try std::optional on the return types from refine? This would avoid using empty arrays as sentinels for "not computed".

[schnellerhase]

Optional return value seems to work as well, in python this just becomes a None value if nothing is set :) - will change the facets as well to this behavior.

[schnellerhase]

Requires prior merge of #3331

[garth-wells]

@schnellerhase could you resolve the merge conflict?