Implemented preprocessing steps for Sentinel-1 data using esa-snappy

[katyagikalo]

Following preprocessing steps were added:

• Orbit file correction
• Thermal noise removal
• Calibration
• Speckle filtering
• Terrain correction

Closes #61

[jsreu]

@katyagikalo : acquisition/clipping/calibration.py can be removed I think. We're not using it anymore, are we?

[katyagikalo]

@katyagikalo : acquisition/clipping/calibration.py can be removed I think. We're not using it anymore, are we?

Yes, I deleted it

[jsreu]

TODOs:

1. Move all esa_snappy imports into separate module s.t. it does not have to be installed if not used.
2. Finish installation guide for esa_snappy in ReadME (macOS user still missing)
3. Add variable to config for choosing DEM (cf. s1_preprocessing.terrain_correction for available values)
4. (Optional) Add bool-variables to config s.t. the user can decide which pre-processing steps he wants to perform. Keep the defaults as True s.t. it performs the ones you mentioned by default.