added 'tax' switch to ResponseSD interface _cmd to match latest specification of dwi2response

[tclose]

The first argument to the latest specification of MRtrix 3's version of dwi2response is now a switch between 5 different methods for generating a diffusion response function. It appears that the ResponseSD interface in NiPype corresponds to the 'tax' option so I have added it into the _cmd attribute of the interface.

It may be desirable to include options for the different methods as well but I think these would work better as separate interfaces in any case.

[coveralls]

Coverage remained the same at 72.325% when pulling 8dcc0f4 on tclose:mrtrix_dwi2response_update into 3ed411b on nipy:master.

[oesteban]

Hi @tclose, would it be too hard to add a traits.Enum input (with default tax) to support the remaining function modes? Would it imply changes in other parameters?

(EDIT: I add the documentation file for convenience here https://github.com/MRtrix3/mrtrix3/blob/9a1dec74ef02231f21106dbed597911ba6fe886e/docs/reference/scripts/dwi2response.rst)

[tclose]

Hi @oesteban, it would not be too hard to add an enum for the differnt switches but then there are about 3-4 options per switch that are specific to that algorithm.

I reckon the best approach would be to create a base class with the common options and then derived classes for each of the switches since they are really different commands under the hood.

[effigies]

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