Context Manager for NamedTemporaryFile

I think we should clean up the use of NamedTemporaryFile in some spots. It has happened before that it was forgotten to remove temp files after use (e.g. in some test suites) and this can be done cleaner using the "with" statement:
```python
from obspy.core.util import NamedTemporaryFile

with NamedTemporaryFile() as tf:
    tf.write("burn after writing")
    # do something useful (hopefully!) with the tempfile
    # tempfile gets closed and deleted after this code block
```