[BUG] invalid indexing of tetrahedron when area is outside of tetrahedral mesh.

[lachlangrose]

Describe the bug
Using PLI if you try and evaluate the mesh outside of the model area it has an indexing problem.
Seems to occur when modelling faults.

packages/LoopStructural/interpolators/structured_tetra.py", line 109, in evaluate_value
values[inside] = np.sum(c[inside,:]*self.properties[prop][tetras[inside,:]],axis=1)
IndexError: index 29086 is out of bounds for axis 0 with size 26880

Need to make sure that inside is working correctly and maybe add a unit test for this.

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Expected behavior
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• OS: [e.g. iOS]
• Python environment [anaconda, python3.6]
• Version [e.g. 22]

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