[mlpack-git] [mlpack/mlpack] Simpler Mapping Object for DatasetMapper; (#758)

[Keon Kim]

Current `maps` object for DatasetMapper can be described as maps of
`map<dimension, pair<bimap<string, MappedType>, numMappings>>` (NumMappings usually being numeric primitive types.)

I think this map can be simplified to two parts.
```
// MapType = maps<dimension, bimap<string, MappedType>>;
MapType maps;
size_t numMappings;
```

and for validation & imputation purposes we could have another mapper (I will call it invalidMaps for now). Which looks like
```
// InvalidMapType = maps<string, std::pair<dimension, point>> 
InvalidMapType invalidMaps;
size_t numInvalidMappings;
```
invalidMaps and maps serve two different purposes.
maps is used as usual (mapping categorical feature to numeric feature).
invalidMaps is used as temporary holder for future imputation. Both x and y coordinates have to be stored in order to track the invalid values, since every invalid values are turned to NaNs.

Ultimately, I think this way we could simplify the use of only one mapping policy instead of many.
What do you think of this idea?



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