BUG: Use load_table in convert_biom_to_table

convert_biom_to_table was using parse_biom_table, but was missing on a lot of
the useful functionality that load_table provides. This was causing a bug in a
QIIME script that tried to load an HDF5 file using convert_biom_to_table.

Note that I moved load_table to biom/util.py but the interface will remain the
same i. e. you can still do:

```python
from biom import load_table
```