Use predictable, boring column names when user fails to specify a real name

This PR changes the fallback name generation logic to create names of the following form:

```python
default_name = '_COLUMN%d_' % column_index
```

Previously, we concocted column names based the on the contents of the Apply expression.  This was buggy (as per #147) and often resulted in column names that were not valid Myrial tokens.

Fixes #147