Analytic functions with blackbody

See #3077 
```
In [15]: c = Column([1.,2.,3.], name='a')

In [16]: c.unit

In [17]: c.unit is None
Out[17]: True
```
So, if I had a table with a column with its unit unset, the code should assume the unit is Angstrom. But if it is, say, `nm`, it should pass the thing through.