Mwrpy

- Fix scan time for BLB files

- Add documentation

- Add exceptions.py
- Avoid mwr-multi processing if only one measurement

- Improve LWP offset correction with humidity dependent threshold
- Adjust method for cloud detection
- Fix rolling mean calculation for plotting

- Skip invalid files

- Update lev2 quality flags and attributes
- Change lev2 indexing to allow combining products retrieved with different elevation angles
- Update thresholds for spectral retrieval