[SYSTEMDS] Fix and cleanup steplm feature selection built-in
[SYSTEMDS] Fix and cleanup steplm feature selection built-in

This patch makes several improvements to the existing steplm built-in
function (correctness and performance):

1) So far, the lm parameters were not correctly passed through to the
actual lm call, which for example rendered tol, reg, and icpt parameters
ineffective (except for icpt=1 which was the only one tested).

2) Cleanup of unnecessarily operations and control flow

3) Converted the main two for loops of greedy model building to parfor
loops (which required a slightly different analysis of the best model).

On a scenario of a dense 10K x 1K input matrix (with convergence after
20 iterations -> ~21000 lm training calls), this patch improved
performance from 103.9s to 14.4s due to much better utilization (with
fewer barriers) of the available 24 virtual cores.

apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml