ENH: uc_from_udic function in fileiobase module.

The uc_from_udic function can be used to create a unit conversion object from
a universal dictionary.  For example:

```python
import nmrglue as ng
dic, data = ng.bruker.read('.')
udic = ng.bruker.guess_udic(dic, data)
uc0 = ng.fileiobase.uc_from_udic(udic, 0)
```