snuffling: added input_directory method
Comes handy when adding stores to a snuffling
Merge pull request #41 from HerrMuellerluedenscheid/snuffling

snuffling: added input_directory method
snuffling: fixed a problem in Snuffling.input_directory()
datacube: improved

- improved drift estimation
- sinc interpolation
- partial read support
- don't rely on leap seconds reported by gps device