afni Allineate caching fails when out_matrix is specified

### Summary
caching of afni's Allineate interface is not handled correctly

### Actual behavior
the interface with the same inputs is rerun when the input `out_matrix` is specified

### Expected behavior
no rerunning with the same inputs

### How to replicate the behavior

### Script/Workflow details
```Python
from nipype.interfaces import afni
from nipype.caching import Memory
memory = Memory('/tmp')
a = memory.cache(afni.Allineate)
out_a = a(in_file='/tmp/anat.nii.gz', reference='/tmp/anat.nii.gz', out_matrix='/tmp/anat_matrix.aff12.1D', out_file='/tmp/anat_allineated.nii.gz')
```
### Platform details:


```console
200104-23:06:04,766 nipype.utils INFO:
	 Running nipype version 1.5.0-dev+gec5e74f36 (latest: 1.4.0)
{'commit_hash': 'ec5e74f36',
 'commit_source': 'repository',
 'networkx_version': '2.4',
 'nibabel_version': '3.0.0',
 'nipype_version': '1.5.0-dev+gec5e74f36',
 'numpy_version': '1.17.4',
 'pkg_path': '/home/salma/CODE/nipype/nipype',
 'scipy_version': '1.3.2',
 'sys_executable': '/home/salma/miniconda3/bin/python',
 'sys_platform': 'linux',
 'sys_version': '3.7.4 (default, Aug 13 2019, 20:35:49) \n[GCC 7.3.0]',
 'traits_version': '5.2.0'}
```

### Execution environment

Choose one
- My python environment outside container

Description

Summary

Actual behavior

Expected behavior

How to replicate the behavior

Script/Workflow details

Platform details:

Execution environment