using shared_ptr comparison for entity_maps keys

[jorgensd]

Summarize the issue

The current code relies on the key of the entity_map being a shared ptr.
This is not safe, as different shared_ptr instances to the same object will give an inequality, ref:
https://cplusplus.com/reference/memory/shared_ptr/operators/
Already been mentioned in: #3262 (comment)

Version

main branch

DOLFINx git commit

f6a86aa

Installation

No response

Additional information

I think we need to use strings to identify the different meshes, as I do not see any scalable way of comparing whole mesh objects (would have to compare all underlying topology and geometry structures, which is very expensive.