merged dataset should accept a list of strings

[FinchPowers]

Blog post about MLDB Hacktoberfest 2016

---

To create a merged dataset, one must specify the dataset with objects containing the "id" key. Eg.:

{
    'type' : 'merged',
    'params' : {
        'datasets' : [{'id' : 'ds1'}, {'id', 'ds2'}]
    }
}

If would be convenient if the ids would also work directly. Eg.:

{
    'type' : 'merged',
    'params' : {
        'datasets' : ['ds1', 'ds2']
    }
}

Context

• merge dataset

Task

• Mldb1921MergeDsStrings test must pass without "expectedFailure".

[nistath]

I can work on this, provided that you can point me to the source code that does the merging at the moment.

[FinchPowers]

Hi @nistath

• builtin/merged_dataset.cc is the merged dataset struct.
• here is the place where the "datasets" key is bound to MergedDatasetConfig::datasets.

[nistath]

To work on top of your existing code, I'll need the method that parses the dataset objects from the json.

[FinchPowers]

It should be here.