add LOOxval
add stdxval inheritance to loo
update looxval example
update splittingstrategy tests
update splittingstrategy tests
Merge pull request #1799 from Saurabh7/loocrossval

add LOOxval
Merge pull request #1797 from Saurabh7/xvalexamples

knn example
Merge pull request #1802 from Saurabh7/xvaltests

update Xvaltests
Small refactoring:
- import modules in python_modular structure HM-SVM examples,
- doc fixes,
- if () {error message} -> REQUIRE in PrimalMosekSOSVM,
- and minor style changes.
Update gitignore with protobuf generated files