Fix issues in the detector data translation in euxfel backend

[egorsobolev]

This fixes issues with detector data translation in euxfel backend:

• convert cells mask to indices, because it is used in numpy.take
• terminate translation if cellId dataset is empty

Also removes old commented code.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (945c95a) 22.53% compared to head (697cad9) 22.52%.

Files	Patch %	Lines
hummingbird/backend/euxfel.py	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #152 +/- ## ========================================== - Coverage 22.53% 22.52% -0.01% ========================================== Files 89 89 Lines 7500 7502 +2 ========================================== Hits 1690 1690 - Misses 5810 5812 +2 ```

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