fix: use keyword bvecs= in gradient_table() calls to suppress dipy deprecation warning

[Devguru-codes]

This PR fixes a dipy deprecation warning in all 6 IAR_LU_ algorithms by converting bvec from a positional argument to the keyword form bvecs=bvec when calling gradient_table().

dipy 1.11.0 emits the following deprecation warning when bvec is passed positionally:

UserWarning: Pass ['bvecs'] as keyword args.
From version 2.0.0 passing these as positional arguments will result in an error.

This will become a hard error in dipy 2.0.0, breaking all 6 IAR_LU algorithms.

Changed 11 call sites across 6 files from:

gtab = gradient_table(self.bvalues, bvec, b0_threshold=0)

to:

gtab = gradient_table(self.bvalues, bvecs=bvec, b0_threshold=0)

Files Changed

File	Lines Changed	Location
IAR_LU_biexp.py	Line 62	__init__ (2 other sites were already fixed)
IAR_LU_modified_mix.py	Lines 61, 93	__init__ and ivim_fit
IAR_LU_modified_topopro.py	Lines 64, 95	__init__ and ivim_fit
IAR_LU_segmented_2step.py	Lines 63, 100	__init__ and ivim_fit
IAR_LU_segmented_3step.py	Lines 63, 102	__init__ and ivim_fit
IAR_LU_subtracted.py	Lines 61, 100	__init__ and ivim_fit

Testing

• All 622 existing test_ivim_fit_saved tests pass (89 skipped, 11 xfailed, 4 xpassed).
• Verified using a reproduction script (bug_2/reproduce_dipy_warning.py) that the warning is no longer emitted after the fix.
• This is a non-breaking change — using the keyword argument produces identical behavior to positional; it only suppresses the deprecation warning and ensures forward compatibility with dipy 2.0.0.

Fixes #156