As required for #874. Need the option to calculate the inverse of a displacement / deformation field.

Alternatively, would this be considered something other than a "conversion" and hence require a different command? E.g. warpcalc (since transformcalc has a -inverse option)?

Would also suggest that the -type option in warpconvert should in fact be a compulsory argument, since the default behaviour when this option is not provided is not entirely obvious.