Add tests to ensure internal type is not mutated except in inplace operations

[jthielen]

Several of the linked issues to #905 were related to the internal type being mutated by NumPy operations (see below). While #905 contained a provisional fix to this in the behavior of __getattr__, no tests to that effect were added. To fully resolve these related issues, tests should be added that NumPy operations do not mutate the internal type. While doing this, it may be a good idea to check non-NumPy operations as well to ensure that the internal type is only changed during explicitly in place operations.

Supersedes #399, #481, #509, #622, #678