MIABIS diagnosis on Observation

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MIABIS diagnosis on Observation.

The MIABIS-on-FHIR IG puts sample-linked diagnoses on Observation
(miabis-observation, value[x].coding[icd-10-who]) and reserves Condition
for diagnoses of the patient the biobank holds no specimen for. Condition.code
is 0..1 and the reference implementation (BBMRI-cz/MoF-library, used by
BBMRI-cz/fhir-module) leaves it empty, so diagnosis, date_of_diagnosis and
diagnosis_age_donor returned zero against MIABIS data. Both placements are
valid, so the snippets now match either, the same way the bbmri flavour matches
its three ICD-10 code systems.

The Diagnosis define and the stratifier basis move to Observation, since a
stratifier takes one population basis and the sample-linked diagnosis is the one
worth breaking down.

Verified against a Blaze store written by fhir-module: diagnosis queries now
return the expected donors where they returned zero, a coded Condition is
still matched, an absent code still returns zero, and the stratifier reports the
ICD-10 codes present.