Add tests for ContinuousObservable

Adds test_continuous_observables.py with 19 tests covering the full  functionality of continuous state management in mesa_signals:

Basic functionality:
- Constant and variable rate depletion over time
- Manual value setting (e.g., energy boost from eating)
- Time-based lazy evaluation

Signal emissions:
- Change notifications when values update
- Threshold crossing detection (upward and downward)
- Multiple threshold management
- No signals when values don't change

Edge cases:
- Zero elapsed time (no spurious updates)
- Multiple accesses at same time point (single calculation)
- Exact threshold values (inclusive boundary handling)
- Negative values (energy going below zero)
- NumPy float compatibility (fixes AttributeError with random values)

Integration:
- Computed properties based on continuous observables
- Batch agent creation with numpy arrays
- Both DEVSimulator (float time) and ABMSimulator (integer steps)
- Wolf-sheep predator-prey scenario with death thresholds

All tests use proper discrete event scheduling through Mesa's simulator rather than manual time manipulation, ensuring realistic simulation behavior.

Author: EwoutH