Do not copy mapping from dependent variable to prediction field in regression analysis

[przemekwitek]

Currently, in case of regression analysis, the mapping is copied from dependent variable to prediction field.
When the dependent variable is of a discrete type (i.e. integer, long, etc.) the prediction field is indexed as a discrete type as well, increasing total prediction error (MSE, R^2).
This PR addresses that by making prediction field mapped dynamically (as float).

Closes https://github.com/elastic/machine-learning-qa/issues/661

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[benwtrent]

Looks good to me :D.

I wonder if we should force the mapping for regression to always be double instead of it being float? I am not sure if the precision loss is a concern or not.

[droberts195]

It's a good point about using double instead of float. float only stores 7 digits of accuracy, so if the dependent variable contained integers greater than 10 million then using float will lose more accuracy than sticking with integer. (integer will lose the fractions from the predictions, but for a number between 10 and 100 million float would make the units and fraction random.) double would be a reasonable compromise as it has 15 digits of accuracy. long has 19, but we'd only suffer the problem at the (unusual) extreme sizes, whereas integers over 10 million are not that uncommon.

[przemekwitek]

I wonder if we should force the mapping for regression to always be double instead of it being float? I am not sure if the precision loss is a concern or not.

This change required bigger changes in the logic that calculates mappings as now Regression imposes constant mapping while Classification copies the mapping from dependent variable.
You might want to take another look @benwtrent and @droberts195.

[benwtrent]

I think this looks good.

There are a bunch of failing tests due to the change :).

[benwtrent]

Suggested change

	* @return {@link List} containing fields for which the mappings should be handled explicitly
	* @return {@link Map} containing fields for which the mappings should be handled explicitly

[przemekwitek]

Done.