Add stochastic weather variability

[jdhoffa]

For example, a cloud front simulation to stress forecast.
PV generation should show realistic variability.

[jdhoffa]

Acceptance criteria proposal:

• Add temporally correlated cloud variability (not IID per step)
• Bound PV multiplier (example: 0.2..1.2)
• Same seed => identical PV sequence
• Different seeds => statistically distinct sequences
• Tests for bounds + determinism