Incomplete io test

[SUZ-tsinghua]

The current io test is incomplete. For example, in test_get_data_into(), we use humanoid.xml to test the correctness of get_data_into(). But humanoid.xml does not contain flex, so copying flex info from d to mjd is under test. A better way will be testing using the xmls in _IO_TEST_MODELS.

[thowell]

@SUZ-tsinghua yes test_get_data_into could be improved by testing all xmls in _IO_TEST_MODELS to validate that more fields are set correctly by io.get_data_into. are you interesting in contributing this improvement? thanks!