OPT.nodes: stores unrelated nodes for the same path

[ppazos]

The problem of OperationalTemplate.nodes is the key is an archetype path.

Then if we want to get nodes by path, two nodes from different archetypes could have the same path and will be returned. Check the OPT below, for the path "/content[archetype_id=openEHR-EHR-OBSERVATION.saturacion_de_oxigeno.v1]/data[at0001]/events[at0002]/data[at0003]/items[at0005]/value" gets ObjectNodes with internal nodes for blood pressure and oxymetry:

https://github.com/ppazos/openehr-basic/blob/master/opts/monitoreo_de_signos_vitales.opt

The other strange thing is that if opt.getNodes() is called using the template path, it returns results, event if the paths used as keys are archetype paths.

Need to debug and check if there is an issue somewhere.

[ppazos]

Retest to check if this commit doesn't fix the issue: 2692f9d

Also create a test for this.