Move assert_allclose to general to avoid a hard dependency on pytest

[szymonlopaciuk]

Description

The module xobjects.test_helpers has so far only been used in testing, however currently the function assert_allclose is imported from in in __init__.py making it part of the main package. The test_helpers module requires pytest, which we don't necessarily want to be a required dependency, but the current implementation effectively makes it so. This PR moves the assert_allclose function to xobjects.general instead to avoid this problem.