Fix CRRA=1.0 division by zero errors across consumption models

[Copilot]

This PR fixes a critical issue where setting CRRA = 1.0 (logarithmic utility) in various consumption models would cause a ZeroDivisionError due to division by (1.0 - CRRA) which equals zero.

Problem

Users attempting to use logarithmic utility by setting CRRA = 1.0 would encounter crashes:

from HARK.ConsumptionSaving.ConsIndShockModel import PerfForesightConsumerType
Example = PerfForesightConsumerType()
Example.CRRA = 1.0
Example.solve()  # ZeroDivisionError: float division by zero

The error occurred in expressions like:

MPCnvrs = self.MPC**(-self.CRRA/(1.0-self.CRRA))

Solution

Added special case handling for CRRA == 1.0 using a small epsilon value (CRRA_safe = 1.0 + 1e-8) as suggested by the maintainers. This approach:

• Prevents division by zero errors
• Maintains numerical stability
• Preserves exact behavior for all other CRRA values
• Includes user warnings about the temporary workaround

Files Modified

• ConsIndShockModel.py: Fixed 5 problematic expressions in solve_one_period_ConsPF, solve_one_period_ConsIndShock, and solve_one_period_ConsKinkedR
• ConsPrefShockModel.py: Fixed 4 expressions in preference shock solvers
• ConsIndShockModelFast.py: Fixed 3 expressions in the fast implementation
• ConsBequestModel.py: Fixed 2 expressions in bequest model solver
• ConsGenIncProcessModel.py: Fixed 1 expression in general income process solver
• test_PerfForesightConsumerType.py: Added test case to prevent regression

Testing

The fix has been thoroughly tested to ensure:

• CRRA=1.0 no longer causes crashes
• All other CRRA values work exactly as before
• Warning messages inform users of the temporary workaround
• Solution quality remains appropriate

This is an interim fix until proper limit calculation for the logarithmic case is implemented.

Fixes #75.

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