ENH: Update default timing precision to 4 digits and add global precision control for Timer and timeit

[Copilot]

This PR enhances QuantEcon's timing functionality by updating the default precision and adding global precision control capabilities for modern timing functions.

Changes Made

1. Updated Default Precision

Timer class and timeit function now default to 4 decimal places instead of 2:

import quantecon as qe
import time

# Before: showed 0.01 seconds  
# After: shows 0.0124 seconds
with qe.Timer():
    time.sleep(0.01234)

2. Added Global Precision Control

Introduced qe.timings.float_precision() function to set precision globally for Timer and timeit:

import quantecon as qe

# Get current precision
current = qe.timings.float_precision()  # Returns 4

# Set precision for Timer and timeit functions globally
qe.timings.float_precision(6)

# Timer and timeit use 6 digits
with qe.Timer(): ...  # 6 decimal places
qe.timeit(func)       # 6 decimal places  

# Deprecated functions maintain original defaults
qe.tac()              # Still uses digits=2 default

# Reset to default
qe.timings.float_precision(4)

3. Affected Functions

The following functions now use the global precision by default:

• Timer class context manager
• timeit() function

The following deprecated functions maintain their original behavior:

• tic(), tac(), toc() functions (continue using digits=2 default)
• loop_timer() function (continues using digits=2 default)

4. Backward Compatibility

Explicit precision parameters still work and override the global setting:

# Still works - explicit precision overrides defaults
with qe.Timer(precision=8): ...
qe.tac(digits=6)

Implementation Details

• Created new quantecon.timings module with precision management
• Global precision only affects Timer and timeit functions (not deprecated functions)
• Deprecated functions maintain original behavior for future deprecation
• Added comprehensive test coverage (27 tests)
• Maintains full backward compatibility
• Input validation ensures precision is a non-negative integer

Testing

All 27 timing tests pass, covering:

• Default precision validation (now 4 for Timer/timeit, still 2 for deprecated functions)
• Global precision get/set functionality
• Input validation
• Integration with Timer and timeit functions
• Explicit parameter override behavior
• Original behavior preservation for deprecated functions

Fixes #804.

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[mmcky]

@copilot I see you have updated

• tic(), tac(), toc() functions
• loop_timer() function

these functions will be deprecated in the future, so let's not change their behaviour. Can we update this PR to just adjust

• Timer() context manager
• timeit() function

[coveralls]

coverage: 92.579% (+0.04%) from 92.539%
when pulling 58f397d on copilot/fix-804
into 08a6188 on main.

[Copilot]

@copilot I see you have updated

• tic(), tac(), toc() functions
  ...

Done! I've reverted the changes to the deprecated functions (tic/tac/toc/loop_timer) to maintain their original behavior with digits=2 default. Only the Timer context manager and timeit function now use the global precision control with 4-digit default.

The changes ensure the deprecated functions maintain backward compatibility exactly as before, while the modern Timer and timeit functions benefit from the enhanced precision control. All 27 timing tests pass.

Commit: 58f397d