Move the following out of ts_salobj into new package ts_utils:

• time functions such as current_tai
• angle functions such as angle_diff and angle_wrap_center
• test utilities such as modify_environ and assert_angles_almost_equal (rename from assertAnglesAlmostEqual)
• make_done_future

Add trivial backward compatibility wrappers to ts_salobj that issues deprecation warnings and then call the new function in ts_utils.

Update ts_salobj to use the new versions (but continue to test the trivial backward compatibility wrappers).