- add id to SubScheme
- add rlinks, rnodes and rsub_schemes which allows you to access to all links, nodes and sub_schemes recursively
- rework from / to json functions to fit the new description
- move some function to static for conveniance

warning: for now the rnodes and rlinks are created once for all. Only a call to _reset can apply modification to them once created

Test moml

Closes #6

See merge request !1