print_attrs executable / multiple inheritance

This closes #213. This work is related to #232, but does not resolve it fully.

**Use**
$ bin/print_attrs -a `entity` `path/to/schema.exp`

**Example**
`$ bin/print_attrs -a si_energy_unit ../data/cd209/part409cdts_wg3n2617mim_lf.exp`

```
Schema ap209_multidisciplinary_analysis_and_design_mim_lf
Entity si_energy_unit
--------------------------------------------------------------------------
 In P21? |       attr name       |        creator        |     detail
--------------------------------------------------------------------------
    Y    |              elements |          derived_unit | 
    N    |                  name |          derived_unit | derived in derived_unit
    *    |            dimensions |            named_unit | derived in si_unit
    Y    |                prefix |               si_unit | 
    Y    |                  name |               si_unit | 
No errors in input
```