API request to train models must support training several models using multiple algorithms instead of just one model with a single algorithm

[georgeslabreche]

E.g. POST request to train a classifier model: /api/v1/training/classifier

Currently the payload looks like this:

{
    "expId": 123,
    "datasetId": 1,
    "algorithm": "LinearBatch"
}

But it should be like this:

{
    "expId": 123,
    "datasetId": 1,
    "algorithms": ["LinearBatch"]
}

And support multiple algorithms:

{
    "expId": 123,
    "datasetId": 1,
    "algorithms": ["LinearBatch", "LinearL1SCD", "NewGLMNET"]
}