[Feature] Minimal IUCLID export (#338)

This is an initial implementation that creates a working minimal .i6z document.
It passes schema validation and can be imported into IUCLID.

Caveat:
IUCLID files target individual compounds.
Pathway is not actually covered by the format.

It can be added in either soil or water and soil OECD endpoints.
**I currently only implemented the soil endpoint for all data.**

This sort of works, and I can report all degradation products in a pathway (not a nice view, but we can report many transformation products and add a diagram attachment in the future).

Adding additional information is an absolute pain, as we need to explicitly map each type of information to the relevant OECD field.
I use the XSD scheme for validation, but unfortunately the IUCLID parser is not fully compliant and requires a specific order, etc.

The workflow is: finding the AI structure from the XSD scheme -> make the scheme validation pass -> upload to IUCLID to get obscure error messages -> guess what could be wrong -> repeat 💣

New specifications get released once per year, so we will have to update accordingly.
I believe that this should be a more expensive feature, as it requires significant effort to uphold.

Currently implemented for root compound only in SOIL:

- Soil Texture 2
- Soil Texture 1
- pH value
- Half-life per soil sample / scenario (mapped to disappearance; not sure about that).
- CEC
- Organic Matter (only Carbon)
- Moisture content
- Humidity

<img width="2123" alt="image.png" src="attachments/d29830e1-65ef-4136-8939-1825e0959c62">
<img width="2124" alt="image.png" src="attachments/ac9de2ac-bf68-4ba4-b40b-82f810a9de93">
<img width="2139" alt="image.png" src="attachments/5674c7e6-865e-420e-974a-6b825b331e6c">

Reviewed-on: enviPath/enviPy#338
Co-authored-by: Tobias O <tobias.olenyi@envipath.com>
Co-committed-by: Tobias O <tobias.olenyi@envipath.com>

epiuclid/builders/reference_substance.py

@@ -0,0 +1,54 @@
+import xml.etree.ElementTree as ET
+
+from epiuclid.serializers.pathway_mapper import IUCLIDReferenceSubstanceData
+
+from .base import (
+    _sub,
+    _sub_if,
+    build_document,
+    document_key,
+)
+
+NS_REFERENCE_SUBSTANCE = "http://iuclid6.echa.europa.eu/namespaces/REFERENCE_SUBSTANCE/10.0"
+ET.register_namespace("", NS_REFERENCE_SUBSTANCE)
+
+
+class ReferenceSubstanceBuilder:
+    def build(self, data: IUCLIDReferenceSubstanceData) -> str:
+        ref = ET.Element(f"{{{NS_REFERENCE_SUBSTANCE}}}REFERENCE_SUBSTANCE")
+
+        _sub(ref, NS_REFERENCE_SUBSTANCE, "ReferenceSubstanceName", data.name)
+        _sub_if(ref, NS_REFERENCE_SUBSTANCE, "IupacName", data.iupac_name)
+        if data.cas_number:
+            inventory = _sub(ref, NS_REFERENCE_SUBSTANCE, "Inventory")
+            _sub(inventory, NS_REFERENCE_SUBSTANCE, "CASNumber", data.cas_number)
+
+        has_structural_info = any(
+            [
+                data.molecular_formula,
+                data.molecular_weight is not None,
+                data.smiles,
+                data.inchi,
+                data.inchi_key,
+            ]
+        )
+        if has_structural_info:
+            structural = _sub(ref, NS_REFERENCE_SUBSTANCE, "MolecularStructuralInfo")
+            _sub_if(structural, NS_REFERENCE_SUBSTANCE, "MolecularFormula", data.molecular_formula)
+
+            if data.molecular_weight is not None:
+                mw = _sub(structural, NS_REFERENCE_SUBSTANCE, "MolecularWeightRange")
+                _sub(mw, NS_REFERENCE_SUBSTANCE, "lowerValue", f"{data.molecular_weight:.2f}")
+                _sub(mw, NS_REFERENCE_SUBSTANCE, "upperValue", f"{data.molecular_weight:.2f}")
+
+            _sub_if(structural, NS_REFERENCE_SUBSTANCE, "SmilesNotation", data.smiles)
+            _sub_if(structural, NS_REFERENCE_SUBSTANCE, "InChl", data.inchi)
+            _sub_if(structural, NS_REFERENCE_SUBSTANCE, "InChIKey", data.inchi_key)
+
+        doc_key = document_key(data.uuid)
+        return build_document(
+            document_key=doc_key,
+            document_type="REFERENCE_SUBSTANCE",
+            name=data.name,
+            content_element=ref,
+        )