Add Jupyter notebook and parametrize example input files

[Copilot]

Adds an example Jupyter notebook at the root directory (mirroring fz-scale) and extends the example input file to demonstrate multi-parameter usage.

Changes

• example_usage.ipynb: New notebook covering plugin installation, input parsing, compilation, parametric runs, and visualization
• examples/Model/input.txt: Extended from single variable to three parameters (x, y, z)
• .fz/calculators/Model.sh: Updated to extract and compute x + y + z; uses awk -v for safe variable passing
• tests/test_plugin.py: Updated assertions for new variables
• README.md: Updated examples to reflect multi-parameter usage

Example input format

# Input parameters
x = ${x}
y = ${y}
z = ${z}

Example API usage

results = fz.fzr(
    input_path="examples/Model/input.txt",
    input_variables={"x": [1.0, 2.0], "y": [1.0], "z": [0.5]},
    model="Model",
    calculators="localhost_Model"
)

Original prompt

like in fz-scale, add a jupyter notebook at root dir, and in examples, input files should be parametrized

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