add threshold to fsl maths

[NicolasGensollen]

This PR proposes to add Threshold to the fsl.maths module.

@ghisvail I might have missed something since the following is not producing the right command line:

>>> from pydra.tasks import fsl
>>> t = fsl.maths.Threshold(threshold=0.0, input_image="toto.nii.gz", output_image="titi.nii.gz")
>>> t.cmdline
'fslmaths toto.nii.gz titi.nii.gz'

Command should be `'fslmaths toto.nii.gz -thr 0.0000 titi.nii.gz'

[ghisvail]

I have added a unit test to the module's doctest. Looks like it's working as intended.

[ghisvail]

I am not able to reproduce this bug anymore 😅