Fix antsRegistration sampling strategy

I did a git repository review, and the samplingStrategy called "Dense" has never been valid.    The documentation is a bit misleading and unclear that to get a "Dense" sampling you use the "None" samplingStrategy.  A documentation clarification change has been submitted to the ANTs main repository.

I believe that this code previously worked because the command line parser in ants had a flaw where any unkonwn sampling strategy would have resulted in the dense sampling. For example previously if you had intended for random sampling, but typed "Tandon" instead, antsRegistration would have run with dense sampling.  Now an error message is given that "Tandon" is not a valid registration strategy.