Retaining legacy API when reversing reference between Grid and SubDomain prevents solve cache from clearing

[EdCaunt]

To implement Functions on SubDomains, the reference between Grid and SubDomain must be reversed such that SubDomains have a parent Grid. However, to avoid breaking existing codes, the legacy API wherein SubDomains are passed to Grid on instantiation must be retained. This creates a circular reference between Grid and SubDomain, preventing the cache from clearing. When the legacy API is removed, tests/test_caching.py::TestMemoryLeaks::test_solve should be updated to reflect this.