Handle multi target inputs

[VesnaT]

• Show error message when input data (or model domain) had more than one target
• Fixes #67

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9da17ff) 93.51% compared to head (65721d6) 93.55%. Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #69 +/- ## ========================================== + Coverage 93.51% 93.55% +0.03% ========================================== Files 16 16 Lines 5029 5057 +28 ========================================== + Hits 4703 4731 +28 Misses 326 326 ```

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