Updates to registration_chain code. Issue #80. This is the first of several check-ins to improve this code.

Main update: Ability to run build model from within the code itself. Yay! (NOTE: We assume everything is input in lsq6 space. This will be fixed in subsequent check-ins.)

Still need to do a lot of clean-up to make the main code more tidy. This will come in subsequent check-ins as well.

More details:
1. The lsq6-->lsq12 transform is concatenated with the lsq12-->nlin xfm within the main run() function. This is so that we never need to specify an additionalXfm in the LongitudinalStatsConcatAndResample call. If we get the final to lsq6 transform from a previous build model run, we won't need to specify it, and I wanted the same behaviour regardless of where the build model is run.
2. Updates to LongitudinalStatsConcatAndResample to make sure that we get the avg --> finalNlin transform from the correct group. We then need to set the currentGroupIndex back to what it was for subsequent i--> i+1 stats calculations.
3. Removed tmp Option group and put directly in the registration chain code, along with adding more options.
4. Added getGroupIndex function, since a lot of times, we'll need to identify a group based on its name (e.g. "nlin") rather than its index. This function grabs the index based on the name and is used to implement the previous point.
5. Added a print statement identifying input and target, if stats module fails, to provide more useful debugging information.