Fix HypreADS for fieldsplit

[connorjward]

Closes #5327

The issue was that V.reconstruct(element=myelem), where V is an indexed function space, was failing to reconstruct the parent space using myelem because it wasn't a mixed element. I think reconstruct is ambiguous for indexed function spaces and should be disallowed. In this case the fix is to drop all parent information by calling collapse().

Feedback on the choice of test is welcome. This was simply lifted from the issue.

[connorjward]

I am still playing around with this, but I think I have a nice improvement to get_function_space etc. I will continue to mess around but some of this will end up going into another PR in main.

[pbrubeck]

Could the same fix be applied to hypre_ams.py and bddc.py?

[connorjward]

I'm about to go on holiday for a week. @pbrubeck you are very welcome to get this over the line. I can fix those other files when I get back but a user is keen to have this fixed so I see no reason to wait.

Done.

[pbrubeck]

I'm about to go on holiday for a week. @pbrubeck you are very welcome to get this over the line. I can fix those other files when I get back but a user is keen to have this fixed so I see no reason to wait.

We don't have to wait, the extension to the other files was trival.