Add unit tests for ALL `InputData` and `Data` classes methods

[kasyanovse]

Tests for correct conditions:

1. Classification tasks:
   1. Binary classification
   2. Multitarget classification
2. Regression task
3. Time series forecasting tasks:
   1. Unidimensional case
   2. Multi_ts case
   3. Multidimensional time series case
4. Multimodal case:
   1. Classification (bi- and multitarget)
   2. Regression
   3. Time series forecasting (unidimensional and multi_ts)