add python extension to  feature class
move feature tests to feature folder, user linear feature by default
move feature title to property, use formula in constructor
require title of formula to init feature
move feature specific tests to feature module
add repr method to feature class
move features tests to feature module, update data reader class
update header parser for DataReader as far as Feature init method was changed
fix duplicated features test
update test according to default linear type for feature
remove extra code
Merge branch 'feature/feature_refactoring_remove_title' into develop
read method for DataReader returns Data boject
Merge branch 'master' into develop
add example to readme
Merge branch 'develop' of github.com:pavlov99/pmll into develop