[ENH] PLS: scores and loadings outputs

[markotoplak]

For me this makes PLS actually useful. I know improvements are possible, for starters supporting multiple targets, but this perhaps closes #283.

[codecov]

Codecov Report

Merging #693 (7d3796e) into master (e6924c0) will increase coverage by 0.05%.
Report is 6 commits behind head on master.
The diff coverage is 96.55%.

❗ Current head 7d3796e differs from pull request most recent head f6c6dea. Consider uploading reports for the commit f6c6dea to get more accurate results

@@            Coverage Diff             @@
##           master     #693      +/-   ##
==========================================
+ Coverage   88.64%   88.69%   +0.05%     
==========================================
  Files          73       73              
  Lines       12399    12454      +55     
==========================================
+ Hits        10991    11046      +55     
  Misses       1408     1408              

[markotoplak]

I am merging this so that it will be easier to test.