MultiApplyTransforms does not support `identity` transform in list

[emdupre]

Discovered in nipreps/fmriprep#2239

Here's a reproducible example, assuming you have a toy 'affine.txt' file locally:

> from niworkflows.interfaces import itk  

> bold_to_t1_transform = itk.MultiApplyTransforms()
> bold_to_t1_transform.inputs.transforms = ['./affine.txt', './affine.txt']  # OK
> bold_to_t1_transform.inputs.transforms = 'identity'  # OK
> bold_to_t1_transform.inputs.transforms = ['identity', './affine.txt']  # Fails

TraitError: The 'transforms' trait of a _MultiApplyTransformsInputSpec instance must be a list of items which are a pathlike object or string representing an existing file or 'identity', but a value of ['identity', './affine.txt'] <class 'list'> was specified.

A transform of ['identity', 'identity'] fails similarly. I can look into this tonight.

[effigies]

I'm taking a shot at this now.

[effigies]

Annoyingly, the bug here is a Nipype problem. If we pass identity to regular ApplyTransforms, we may need to patch that, too.

[oesteban]

There were concerns that led to not implementing the identity because it's not documented. But yes, this pertains in nipype's proper.