(LambdaMART) File format to run example

[rzykov]

Could you provide file formats to run LambdaMARTRunner.scala? Or files itself?
It would be better to support this defacto standart: https://arxiv.org/ftp/arxiv/papers/1306/1306.2597.pdf

[cmacdonald]

I'm also mystified: LambdaMARTRunner.loadTrainingData() doesn't seem to be designed to parse a LETOR like format.

[witgo]

CC @hucheng