converttz should check header and `fromtz`

If the header says something like:

```
Datetime:UTC,variable1
2001-01-01T00:00,1.2
2001-01-01T01:00,1.3
2001-01-01T02:00,43.4
...
```

Then
`tstoolbox converttz EST America/New_York
`
should fail since "UTC" in the header doesn't match the `fromtz` value "EST" in the function call.