PEP8 formatting
PEP8
PEP8
Calculation buttons now enable/disable when appropriate
Fixed bug with tempfile deletion

Tempfiles were being deleted upon exit of scope (duh).
Now we pass them along with filename created and make sure we cleanup
when plugin disconnects (e.g., on QGIS shutdown or plugin unload).
Deletion attempted first with GDAL (to cover metadata/associated files)
and then with tempfile