Extend assembly with find; fix remove and add on nested assemblies

[adam-urbanczyk]

• find(self, name: str) -> list[str]
• remove(self, list[str]) overload
• Fix remove for nested assmblies, i.e. remove all references
• Fix add used on already owned subassembly: assy.subassy.add(...) does not currently add to the parent.
• Tests
• Docs

Related to #2014 , CC: @shimwell

[shimwell]

could we have something that just returns the names of solids in the assembly. currently doing [i for i,j in assembly.traverse][:-1] to avoid getting the name of the assembly itself. perhaps assembly.names

[shimwell]

assembly.rename(old_name:str, new_name:str) would also be useful if the step file arrives with incorrect names, it would save remaking it