[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>

pyproject.toml

@@ -31,6 +31,8 @@ dependencies = [
     "setuptools>=80.8.0",
     "nh3==0.3.2",
     "polars==1.35.1",
+    "xmlschema>=3.0.0",
+
 ]
 
 [tool.uv.sources]
@@ -139,6 +141,7 @@ collectstatic = { cmd = "uv run python manage.py collectstatic --noinput", help
 frontend-test-setup = { cmd = "playwright install", help = "Install the browsers required for frontend testing" }
 
 [tool.pytest.ini_options]
+DJANGO_SETTINGS_MODULE = "envipath.settings"
 addopts = "--verbose --capture=no --durations=10"
 testpaths = ["tests", "*/tests"]
 pythonpath = ["."]
@@ -155,4 +158,5 @@ markers = [
     "frontend: Frontend tests",
     "end2end: End-to-end tests",
     "slow: Slow tests",
+    "iuclid: IUCLID i6z export tests",
 ]