Custom quantization borders and missing value modes

Custom quantization borders and the method for processing missing values for the dataset numerical features.

Each line contains information regarding a single border and optionally the missing values mode settings for the corresponding feature.
Missing value modes are output for a feature if the following conditions are met at the same time:
- The chosen missing value mode for the feature differs from Forbidden.
- Missing values are present in the dataset.
The global missing value mode is specified in the --nan-mode (nan_mode) training parameter and can be overridden in the Custom quantization borders and missing value modes input file.

Format of a single line:

<zero-based feature ID><\t><border value><\t><missing value mode>

0<\t>0.25
0<\t>0.75
2<\t>0.3<\t>Max
2<\t>0.85<\t>Max