asfreq_info has redundant fields / poor comments
looking at the definition for the `asfreq_info` struct in `period.h`, I see the following lines:

```
int from_a_year_end; // month the year ends on in the "from" frequency
int from_q_year_end; // month the year ends on in the "from" frequency

int to_a_year_end;   // month the year ends on in the "to" frequency
int to_q_year_end;   // month the year ends on in the "to" frequency
```

As far as I can tell, the comments are wrong and a union could have been used for these values as they are never simultaneous set. 

Thoughts?
