Revise InputMultiObject of the BSpline interface

[oesteban]

See https://github.com/nipreps/sdcflows/pull/346/files#r1294641020

[effigies]

Noting that this is specifically about these inputs, since following this link is a little messy:

sdcflows/sdcflows/interfaces/bspline.py

Lines 302 to 311 in 50ace2a

	fmap2data_xfm = InputMultiObject(
	File(exists=True),
	desc="the transform by which the target EPI can be resampled on the fieldmap's grid.",
	xor="data2fmap_xfm",
	)
	data2fmap_xfm = InputMultiObject(
	File(exists=True),
	desc="the transform by which the fieldmap can be resampled on the target EPI's grid.",
	xor="fmap2data_xfm",
	)

Seems confusing making these MultiObjects, since the first is always being used and the rest discarded - and we should specify that these need to be in ITK format.

This is for compatiblity with the nipype interface. We probably want not to drop linear transforms after the first, although it feels like a rabbit hole.

So, rather than operating here, I think we want to open an issue to think about how a list here should be managed later.