Assert 2.1.0 broke ASSERT_PARALLEL_CALLBACKS

[bonachea]

The 2.1.0 release inadvertently broke the ASSERT_PARALLEL_CALLBACKS feature.

The problem was introduced in this commit, which incorrectly removed the assert_this_image and assert_error_stop callback pointers from the public interface.

[bonachea]

Fixed in #58

[bonachea]

This workflow shows berkeleylab/caffeine:main @ 899ba80b (code identical to Caffeine v0.5.2) successfully running against berkeleylab/assert:main @ de4c9ea (after the merge of #58). This demonstrates the ASSERT_PARALLEL_CALLBACKS feature that was broken in version 2.1.0 (the subject of this issue) has now been fully repaired.

CC: @rouson