User gets unsupported matplotlib backend.

It looks like the code to select the 'Agg' backend for non-interactive use

https://github.com/sot/starcheck/blob/master/starcheck/calc_ccd_temps.py#L26

doesn't get hit in the standard use case (just importing get_ccd_temps), so a user-specified or default interactive backend is set.  SOT MP seems to get GTKAgg which breaks with
```
ERROR: exceptions.RuntimeError: Cannot get window extent w/o renderer at line 14
```


https://github.com/sot/starcheck/blob/master/starcheck/calc_ccd_temps.py#L26