Replace list+ndarray concatenation with numpy operations in tutorials

[Copilot]

The pattern [-1] + revenue_locations mixes Python list concatenation with numpy arrays, creating type inconsistency and obscuring intent.

Changes

• Replaced [-1] + array_name with np.insert(array_name, 0, -1) in both tutorials
• Updated 4 instances in hillstrom.rst
• Updated 4 instances in oregon.rst

Example

# Before
locations=[-1] + revenue_locations  # Returns list

# After  
locations=np.insert(revenue_locations, 0, -1)  # Returns ndarray

Both produce functionally equivalent results, but the numpy operation maintains type consistency and clearly expresses the intent to prepend -1 to the array.

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