By default, assert uses `THIS_IMAGE()` in multi-image mode while
composing assertion output, and invokes `ERROR STOP` to print the
assertion and terminate execution.

The ASSERT_PARALLEL_CALLBACKS preprocessor flag enables the client to replace
the default use of these two Fortran features with client-provided callbacks.
To use this feature, the client must build the library with `-DASSERT_PARALLEL_CALLBACKS`,
and then at startup set the `assert_this_image` and `assert_error_stop`
procedure pointers to reference the desired callbacks.