Update FSL Merge to Support TR Parameter

The issue of a missing TR paramerter for FSL Merge was raised on the mailing list by Falko. I've modified the input_spec `dimension` input to take "tr" as an option, and added a `tr` input to specify the TR.

One issue: with "tr" listed as the value for `dimension` the "tr" input is required, but I wasn't sure how to make this input Mandatory only when `dimension` is set to "tr". I included info in the docs and examples to explain this.