Improve usability of instance/mapper

This PR change the way instance/mapper works : 
- Instance are not immutable anymore
- Instance hold (transient) references to input/ouput mappers
- Mapper does have feature index anymore
- Mapping type renamed : FIXED_FEATURES : NAMED, USER_SELECTION -> LISTED
- Output/Input mapper are merged. There is now 2 mapper : NamedFeaturesMapper and ListedFeaturesMapper
- Declaration of stream's mapping type and features moved from @Stream -> @NameMapper or @ListMapper

Fix #50