[Enhancement] Add axisymmetric compatibility to log-conform-viscoelastic-3D

## Overview
Currently, the code lacks axisymmetric (axi) compatibility. While this has been marked as non-critical and non-urgent in the codebase, documenting the required changes and considerations could be valuable for future reference or if implementation becomes necessary.

## Current Status
- Code explicitly prevents axi usage with an error message
- Alternative implementations are available in:
- `log-conform-viscoelastic-scalar-2D.h` (scalar formulation)
- `log-conform-viscoelastic.h` (more efficient implementation)

## Required Changes

### 1. Tensor Compatibility
- [ ] Enforce tensor compatibility throughout the codebase
- [ ] Implement consistent usage of `foreach_dimensions` for tensor operations
- [ ] Review and update tensor calculations for axi compatibility

### 2. Metric Terms Implementation
- [ ] Review current usage of metric terms (`cm` and `fm`)
- [ ] Implement missing metric terms where needed
- [ ] Ensure proper scaling in cylindrical coordinates

### 3. Code Structure
- [ ] Update error handling to support axi mode
- [ ] Add conditional compilation for axi-specific code
- [ ] Update documentation to reflect axi compatibility

## Technical Considerations
- Impact on performance when using metric terms
- Compatibility with existing 3D implementations
- Potential conflicts with current tensor calculations
- Testing requirements for axi mode

## Notes
- This enhancement is marked as non-critical and non-urgent
- Current workaround: Use alternative implementations mentioned above
- Implementation should maintain code readability

## Labels
- enhancement
- documentation
- help wanted
- good first issue

## Related Files
- `log-conform-viscoelastic-3D.h`
- `log-conform-viscoelastic-scalar-2D.h`
- `log-conform-viscoelastic.h`