Fixed bug that made sanity check on acqparams.txt reject synb0-DISCO use

There is a sanity check on readout times in the --datain/--acqp files. I recently coordinated that with eddy's sanity check, to ensure they have the same view of what is sane. But that meant that registering to a distortion free image no longer worked. And that is a valid thing to do, for example in the context of synb0-DISCO. I have now fixed it so that it allows one volume to have a readout time of zero, or almost zero.